Labour MP Mary Creagh arrived at the scene shortly after the incident.
She told the BBC: "When we were on our way to vote there were armed police officers standing at the escalator.
"As I got off the escalator I saw people running towards me. Penny Mordaunt [another MP] ran up and said 'shots have been fired, we need to get out now'.
"We had no choice but to go out of the revolving doors one at a time - I shouted to the security guard 'you've got to open the doors' and he then opened them.
"I found myself in Westminster Tube station, I thought the one thing we don't want is tourists wandering in with their children into this situation.
"I went into the control room and told the controller 'there's been what we think is a terror attack on the Palace of Westminster and I said 'you need to shut the station now'."
River closed between Vauxhall Bridge and Embankment
The Port of London Authority have said the River Thames has been closed between Vauxhall Bridge and Embankment.
Severe disruption: A4 London eastbound
BBC News Travel
A4 London eastbound severe disruption, between St James's Street and Trafalgar Square.
A4 London - A4 Pall Mall in Westminster closed eastbound between the St James's Street junction and the Trafalgar Square junction, because of a security alert.
To report traffic and travel incidents dial 0330 123 0184 at any time
Lifeboat launched twice after attack
The RNLI has confirmed the Tower lifeboat was requested to launch twice to help deal with the attack.
It was first launched at 14:25 in connection with the incident then again at 15:55.
The boat returned to the station at 17:10.
Pupils who posed for picture before terror attack 'are safe'
Alex Homer
BBC Local Live
The head teacher of Holy Family Catholic Primary School in Small Heath, Birmingham, Peter Foley, has reiterated the pupils who had been visiting Parliament are safe.
The school Twitter account published a photo of the children there shortly before the terrorist incident on Westminster Bridge unfolded.
He said: “All the children on the school trip are safe and their parents have been informed of the situation. The deputy head teacher is with the children and is providing me with regular updates.”
British Transport Police tell me they have extra officers out tonight across the whole British rail network, not just London, in order "to reassure" people.
No numbers have been given and they aren't clear if they will be armed or not.
To be clear, they say it's just to reassure people, not because of a specific threat.
But you could see a bigger presence going home tonight.
Officer stabbed in Westminster 'died at the scene'
Three French high school students injured - French media
Three French high school students are among those who have been hurt in the attack, French media are reporting.
They were part of a visiting group from a school in the town of Concarneau in Brittany.
The students were hit by the car that drove into pedestrians on Westminster Bridge. One of the students is in a critical condition,
Ouest France newspaper reported
. Another has serious injuries and there is no information on the condition of the third student, the newspaper said.
After the incident the students were evacuated onto a boat and taken back to the hostel they were staying at,
Le Telegramme newspaper reported
.
Poilce Statement: This is a terrorist incident
BBC
Full statement from Metropolitan Police Commander, BJ Harrington:
"Since 2.40 pm this afternoon
the [Metropolitan Police Service] has responded to an incident in the area of Parliament Square.
The senior National
co-ordination has declared this is a terrorist incident.
Although we remain open-minded to the motive, a full counter-terrorism investigation is already underway. This is led by the Met counter-terrorism command.
At this stage I will confirm what we know has happened but I will not speculate. We received a number of different reports which included a person in the river, a car in collision with pedestrians and a man armed with a knife. Officers were already in that location as part of routine policing but immediately additional officers were sent to the scene and that included firearms officers. We are working closely with the London ambulance service and the London fire brigade ...
We know there are a number of casualties, including police officers, but at this stage we cannot confirm numbers or the nature of these injuries ...
The acting commissioner, Craig Mackey, is being treated as a significant witness as he was at the scene when the incident started. Whilst he is not injured, it would be inappropriate for him to be here to talk about this incident at this stage.
Our thoughts and his thoughts are with all those involved and responding to that incident this evening. I would stress that if anyone has information about today’s incident they are urged to call 0800789321 and I want to stress that if the public have any information or see anything suspicious to dial 999 immediately."
'Our priority is London'
Getty Images
"I would absolutely stress to the public our number one priority, working with our partners, is to keep London safe. If people have information please dial the number - 0800 789321."
People get off the London Eye after Westminster attack
Sightseers held on the London Eye for safety are now getting off, say the attractions bosses.
"Victoria Street at the junction with Storeys Gate has been closed towards Parliament Square. Lambeth Palace Road has been closed northbound at the roundabout by Albert Embankment. Traffic is being diverted by the Police. Traffic is also queuing southbound on York Road with tailbacks approaching Leake Street. Traffic southbound is slow moving with tailbacks to Downing Street. Millbank is closed to traffic northbound at Lambeth Bridge and traffic is being diverted. Drivers are advised to use an alternative route where possible. "
Affected roads include:
A3211 (SW1A) both directions
A3212 (SW1P, SW1A) both directions
A302 Bridge Street (SE1,SW1P,SW1A) both directions
Canon Row (SW1A) all directions
Youngsters in House of Parliament during terror attack
Young people from Hammersmith Boxing Club were at the Houses of Parliament when shots were fired,
Get West London has reported.
The site is running live video of the aftermath in London.
'At least 10 patients' treated on Westminster Bridge
PA
London Ambulance Service deputy director of operations Pauline Cranmer said:
“We can confirm we have treated at least 10 patients on Westminster Bridge and have put a number of hospitals on alert as we continue to respond to this incident.
“We were called at 2.40pm today, with the first ambulance crew arriving within six minutes.
“We have sent a number of resources to the scene including ambulance crews, London’s Air Ambulance and our Hazardous Area Response Team.
“We are working closely with other members of the emergency services at the scene, with our priority being to ensure patients receive the medical help they need as quickly as possible.
“We have declared a major incident and our priority is to assess patients and ensure that they are treated and taken to hospital as soon as possible.
“As we are very busy dealing with this incident, we would ask the public to only call us in a genuine emergency.”
Police advice: 'Remain calm and alert'
Police are advising people to avoid the area and to remain "calm... alert and vigilant".
Eyewitness 'saw bodies' after Westminster terror incident
Steve Voake was on Westminster Bridge when a car mowed down pedestrians.
Statement from Mayor of London
“There
has been a serious incident near to Parliament Square this afternoon which is
being treated as a terrorist attack until the police know otherwise.
"I have spoken to the Acting Commissioner. The Metropolitan Police Service
is dealing with the incident and an urgent investigation is underway. My
thoughts are with those affected and their families.
I would like to express my thanks to the police and emergency services who work so hard to keep us safe and show tremendous bravery in exceptionally difficult circumstances
The BBC understands Tobias Ellwood, Foreign Office Minister, attempted to resuscitate an injured police officer before the London Air Ambulance arrived.
An image of Mr Ellwood, who served with the British Army with the Royal Green Jackets, on social media appears to show blood on his clothing.
@Hugh_Dickinson
'Use 999 for emergencies only'
The Met Contact Centre is asking people to use 999 for emergencies only.
Live Reporting
All times stated are UK
Get involved
In pictures: 'Terrorist incident' at Westminster
Four people, including a police officer and an attacker, are dead and 20 others have been injured in a "terrorist incident" in Westminster.
Good evening from BBC London Live
Updates on BBC London Live covering the attacks at Westminster have now ended for the day but our UK colleagues will be bringing you the very latest news and information throughout the night.
What we know so far:
Westminster attack: latest updates
MP tells of shutting Westminster Tube
Labour MP Mary Creagh arrived at the scene shortly after the incident.
She told the BBC: "When we were on our way to vote there were armed police officers standing at the escalator.
"As I got off the escalator I saw people running towards me. Penny Mordaunt [another MP] ran up and said 'shots have been fired, we need to get out now'.
"We had no choice but to go out of the revolving doors one at a time - I shouted to the security guard 'you've got to open the doors' and he then opened them.
"I found myself in Westminster Tube station, I thought the one thing we don't want is tourists wandering in with their children into this situation.
"I went into the control room and told the controller 'there's been what we think is a terror attack on the Palace of Westminster and I said 'you need to shut the station now'."
Police - 'We are not afraid'
River closed between Vauxhall Bridge and Embankment
The Port of London Authority have said the River Thames has been closed between Vauxhall Bridge and Embankment.
Severe disruption: A4 London eastbound
A4 London eastbound severe disruption, between St James's Street and Trafalgar Square.
A4 London - A4 Pall Mall in Westminster closed eastbound between the St James's Street junction and the Trafalgar Square junction, because of a security alert.
To report traffic and travel incidents dial 0330 123 0184 at any time
Lifeboat launched twice after attack
The RNLI has confirmed the Tower lifeboat was requested to launch twice to help deal with the attack.
It was first launched at 14:25 in connection with the incident then again at 15:55.
The boat returned to the station at 17:10.
Pupils who posed for picture before terror attack 'are safe'
Alex Homer
BBC Local Live
The head teacher of Holy Family Catholic Primary School in Small Heath, Birmingham, Peter Foley, has reiterated the pupils who had been visiting Parliament are safe.
The school Twitter account published a photo of the children there shortly before the terrorist incident on Westminster Bridge unfolded.
He said: “All the children on the school trip are safe and their parents have been informed of the situation. The deputy head teacher is with the children and is providing me with regular updates.”
Two killed in Westminster 'terror' attack
Police officer is stabbed in Parliament and a women among several people struck by car on nearby bridge.
Read moreView from Waterloo Bridge
Tom Edwards
Transport Correspondent, BBC London
I'm on Waterloo Bridge where not much traffic is moving because of the road closures in place.
No boats are moving on this part of the River Thames.
You can also see people on the London Eye but it is not moving.
Met police praised
Prominent police blogger, former Met borough commander and police negotiator tweets:
Former newspaper political editor tweets:
The Church of England shares a prayer for Westminster
The Church of England has shared a prayer for Westminster on twitter.
Extra officers across rail network
Richard Westcott
Transport correspondent
British Transport Police tell me they have extra officers out tonight across the whole British rail network, not just London, in order "to reassure" people.
No numbers have been given and they aren't clear if they will be armed or not.
To be clear, they say it's just to reassure people, not because of a specific threat. But you could see a bigger presence going home tonight.
Officer stabbed in Westminster 'died at the scene'
A police officer stabbed in the attack at the Palace of Westminster earlier died at the scene, says the minister who gave mouth-to-mouth resuscitation .
'Two attackers in vehicle on Westminster Bridge'
BBC News Home Affairs Correspondent tweets...
Three French high school students injured - French media
Three French high school students are among those who have been hurt in the attack, French media are reporting.
They were part of a visiting group from a school in the town of Concarneau in Brittany.
The students were hit by the car that drove into pedestrians on Westminster Bridge. One of the students is in a critical condition, Ouest France newspaper reported . Another has serious injuries and there is no information on the condition of the third student, the newspaper said.
After the incident the students were evacuated onto a boat and taken back to the hostel they were staying at, Le Telegramme newspaper reported .
Poilce Statement: This is a terrorist incident
Full statement from Metropolitan Police Commander, BJ Harrington:
"Since 2.40 pm this afternoon the [Metropolitan Police Service] has responded to an incident in the area of Parliament Square.
The senior National co-ordination has declared this is a terrorist incident.
Although we remain open-minded to the motive, a full counter-terrorism investigation is already underway. This is led by the Met counter-terrorism command.
At this stage I will confirm what we know has happened but I will not speculate. We received a number of different reports which included a person in the river, a car in collision with pedestrians and a man armed with a knife. Officers were already in that location as part of routine policing but immediately additional officers were sent to the scene and that included firearms officers. We are working closely with the London ambulance service and the London fire brigade ...
We know there are a number of casualties, including police officers, but at this stage we cannot confirm numbers or the nature of these injuries ...
The acting commissioner, Craig Mackey, is being treated as a significant witness as he was at the scene when the incident started. Whilst he is not injured, it would be inappropriate for him to be here to talk about this incident at this stage.
Our thoughts and his thoughts are with all those involved and responding to that incident this evening. I would stress that if anyone has information about today’s incident they are urged to call 0800789321 and I want to stress that if the public have any information or see anything suspicious to dial 999 immediately."
'Our priority is London'
People get off the London Eye after Westminster attack
Sightseers held on the London Eye for safety are now getting off, say the attractions bosses.
Woman recovered from river
The Port of London Authority has confirmed that a seriously injured woman has been recovered from the Thames river.
Spoken Martin Garside said: "A female member of the public was recovered alive from the water, but with serious injuries.
"She has been brought ashore and is undergoing urgent medical treatment. The working assumption is that she fell or jumped from the bridge".
He added that the river is completely closed to vessels between Vauxhall Bridge and the Embankment
Staff departing Ministry of Defence building
Anna O'Neill
BBC Radio London reporter
Staff are now leaving the Ministry of Defence building on Horse Guards Avenue.
Special service in Southwark for victims
Road closures in Westminster
Details from Transport for London
Westminster Bridge is closed westbound at York Road.
There is no traffic access to Parliament Square.
Victoria Embankment is closed southbound at Northumberland Avenue.
Northumberland Avenue eastbound from Trafalgar Square is also blocked.
Whitehall has also been closed southbound at the junction with Trafalgar Square by the police.
TfL adds :
"Victoria Street at the junction with Storeys Gate has been closed towards Parliament Square. Lambeth Palace Road has been closed northbound at the roundabout by Albert Embankment. Traffic is being diverted by the Police. Traffic is also queuing southbound on York Road with tailbacks approaching Leake Street. Traffic southbound is slow moving with tailbacks to Downing Street. Millbank is closed to traffic northbound at Lambeth Bridge and traffic is being diverted. Drivers are advised to use an alternative route where possible. "
Affected roads include:
A3211 (SW1A) both directions
A3212 (SW1P, SW1A) both directions
A302 Bridge Street (SE1,SW1P,SW1A) both directions
Canon Row (SW1A) all directions
Youngsters in House of Parliament during terror attack
Young people from Hammersmith Boxing Club were at the Houses of Parliament when shots were fired, Get West London has reported.
The site is running live video of the aftermath in London.
'Woman pulled alive from Thames'
The Daily Telegraph
'At least 10 patients' treated on Westminster Bridge
London Ambulance Service deputy director of operations Pauline Cranmer said:
“We can confirm we have treated at least 10 patients on Westminster Bridge and have put a number of hospitals on alert as we continue to respond to this incident.
“We were called at 2.40pm today, with the first ambulance crew arriving within six minutes.
“We have sent a number of resources to the scene including ambulance crews, London’s Air Ambulance and our Hazardous Area Response Team.
“We are working closely with other members of the emergency services at the scene, with our priority being to ensure patients receive the medical help they need as quickly as possible.
“We have declared a major incident and our priority is to assess patients and ensure that they are treated and taken to hospital as soon as possible.
“As we are very busy dealing with this incident, we would ask the public to only call us in a genuine emergency.”
Police advice: 'Remain calm and alert'
Police are advising people to avoid the area and to remain "calm... alert and vigilant".
In pictures: Westminster terror attack
Westminster Abbey to hold prayers
Westminster Abbey is closed for the rest of the day, but is still planning to hold evening prayer for anyone in the area.
London attractions closed after attack
Several attractions have been closed with visitors kept inside following the attack. These include:
Witness describes scene on bridge
Statement from Mayor of London
“There has been a serious incident near to Parliament Square this afternoon which is being treated as a terrorist attack until the police know otherwise.
"I have spoken to the Acting Commissioner. The Metropolitan Police Service is dealing with the incident and an urgent investigation is underway. My thoughts are with those affected and their families.
Man arrested for trying to breach police cordon
Stay away from terror scenes say police as exclusion zone expanded
Evening Standard
Londoners were told to stay away from a swathe of central London after a terror attack in Westminster, the Evening Standard has reported.
Minister 'tried to resuscitate police officer'
The BBC understands Tobias Ellwood, Foreign Office Minister, attempted to resuscitate an injured police officer before the London Air Ambulance arrived.
An image of Mr Ellwood, who served with the British Army with the Royal Green Jackets, on social media appears to show blood on his clothing.
'Use 999 for emergencies only'
The Met Contact Centre is asking people to use 999 for emergencies only.
Two ambulances 'screech away' as cordon moved back
Anna O'Neill
BBC Radio London reporter
Two ambulances have just screeched up Whitehall away from the scene as the police cordon and the public are moved further back.
Facebook activates Safety Check
Facebook has activated its Safety Check service for people in London to let friends and family know that they are safe.
It was used after the 2016 Brussels bombings and the lorry attack in Nice.
Labour leader tweets
Jeremy Corbyn, Leader of the Opposition and Islington MP, has tweeted:
Eyewitness 'hid behind concrete'