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Four people, including an armed police officer and a man believed to be the attacker, have died in a terrorist incident near the UK's Houses of Parliament, Scotland Yard has said.
A woman was among several pedestrians struck by a car on Westminster Bridge, before it crashed into railings.
Police said an officer protecting Parliament was stabbed by an attacker, who was shot by police.
At least 20 people were injured, including three other officers.
Acting Deputy Commissioner and Head of Counter Terrorism at the Metropolitan Police, Mark Rowley, said a major terrorist investigation was under way.
He said the attack started when the car was driven over Westminster Bridge hitting and injuring a number of members of the public - including the three police officers, who were on their way back from a commendation ceremony.
He said after the car crashed, a man armed with the knife "continued the attack and tried to enter Parliament".
Prime Minister Theresa May is to chair a meeting of the government's emergency Cobra committee later.
The French Prime Minister Bernard Cazeneuve said three French school pupils were among the injured and offered "solidarity with our British friends, and full support" for the wounded students and their families.
The Port of London Authority said a woman has been pulled alive from the River Thames near the bridge and was being treated for serious injuries.
Analysis
Image copyright PA By Dominic Casciani, home affairs correspondent
The incident outside Westminster is exactly the kind of scenario that security chiefs have been planning for.
It looks like the type of attack jihadis have wanted to carry out in Britain - namely attacking people with a vehicle and taking on the security forces with knives.
In the security services' jargon this is known as a "marauding attack" and is the hardest type of terrorist incident to predict and defend against. That means casualties, as we have seen in Nice and elsewhere, are inevitable.
But what matters just as much is how the police then respond.
Armed police at Parliament were able to stop the attacker. Within minutes, Westminster was flooded with more armed officers, including counter-terrorism specialists.
Inside Scotland Yard, teams of detectives began working on the next critical phase - establishing the suspect's movements, whether he acted alone and, in tandem with their colleagues on the street, making sure London is as secure as it can be in light of these awful events.
Westminster remains locked down and it will remain so until Scotland Yard is certain the threat has been contained.
MPs said they had heard three or four gunshots and staff inside Parliament were told to stay inside their offices.
Commons Leader David Lidington told MPs the "alleged assailant was shot by armed police".
Image copyright AP Image caption A man believed to be the suspect received medical treatment Shortly after the incident, a Downing Street source confirmed that Mrs May was safe.
The prime minister was seen being ushered into a silver Jaguar car as what sounded like gunfire rang out at Parliament during the incident.
The White House said later that Mrs May had spoken to President Donald Trump about the attack.
'Screams and commotion'
Parliament was put into lockdown shortly after the incident at about 14:40 GMT.
Tom Peck, political editor for the Independent, tweeted: "There was a loud bang. Screams. Commotion. Then the sound of gunshots. Armed police everywhere."
Media captionSteve Voake, eyewitness: "I tried to stop people coming on to the bridge" Press Association political editor Andrew Woodcock witnessed the scenes unfolding from his office window overlooking New Palace Yard.
"I heard shouts and screams from outside and looked out, and there was a group of maybe 40 or 50 people running round the corner from Bridge Street into Parliament Square.
"They appeared to be running away from something.
"As the group arrived at the Carriage Gates, where policemen are posted at the security entrance, a man suddenly ran out of the crowd and into the yard.
"He seemed to be holding up a long kitchen knife.
"I heard what sounded like shots - I think about three of them - and then the next thing I knew there were two people lying on the ground and others running to help them.
"Armed police were quickly on the scene and I heard them shouting to people to get out of the yard."
'Mowed down'
An eye witness, Radoslaw Sikorski, a senior fellow at Harvard's Centre for European Studies, posted a video to Twitter showing people lying injured in the road on Westminster Bridge.
Image copyright Alison Baskerville Image caption Parliament went into lockdown after the attack Image copyright AP Image copyright AP He wrote: "A car on Westminster Bridge has just mowed down at least 5 people."
Scotland Yard said it was called to a firearms incident on the bridge amid reports of several people injured.
Transport for London said Westminster underground station has been shut at the police's request, and buses diverted.
Media captionAerial views of Houses of Parliament Mr Lidington said: "It seems that a police officer has been stabbed, that the alleged assailant was shot by armed police.
"An air ambulance is currently attending the scene to remove the casualties.
"There are also reports of further violent incidents in the vicinity of the Palace of Westminster but I hope colleagues on all sides will appreciate that it'd be wrong of me to go into further details until we have confirmation from the police and from the House security authorities about what is going on."
A woman was among several pedestrians struck by a car on Westminster Bridge, before it crashed into railings.
Police said an officer protecting Parliament was stabbed by an attacker, who was shot by police.
At least 20 people were injured, including three other officers.
Acting Deputy Commissioner and Head of Counter Terrorism at the Metropolitan Police, Mark Rowley, said a major terrorist investigation was under way.
He said the attack started when the car was driven over Westminster Bridge hitting and injuring a number of members of the public - including the three police officers, who were on their way back from a commendation ceremony.
He said after the car crashed, a man armed with the knife "continued the attack and tried to enter Parliament".
Prime Minister Theresa May is to chair a meeting of the government's emergency Cobra committee later.
- Eyewitness accounts
- How the news broke
- Video: Aftermath on the bridge
- 'Terror incident': In pictures
The French Prime Minister Bernard Cazeneuve said three French school pupils were among the injured and offered "solidarity with our British friends, and full support" for the wounded students and their families.
The Port of London Authority said a woman has been pulled alive from the River Thames near the bridge and was being treated for serious injuries.
Analysis
Image copyright PA By Dominic Casciani, home affairs correspondent
The incident outside Westminster is exactly the kind of scenario that security chiefs have been planning for.
It looks like the type of attack jihadis have wanted to carry out in Britain - namely attacking people with a vehicle and taking on the security forces with knives.
In the security services' jargon this is known as a "marauding attack" and is the hardest type of terrorist incident to predict and defend against. That means casualties, as we have seen in Nice and elsewhere, are inevitable.
But what matters just as much is how the police then respond.
Armed police at Parliament were able to stop the attacker. Within minutes, Westminster was flooded with more armed officers, including counter-terrorism specialists.
Inside Scotland Yard, teams of detectives began working on the next critical phase - establishing the suspect's movements, whether he acted alone and, in tandem with their colleagues on the street, making sure London is as secure as it can be in light of these awful events.
Westminster remains locked down and it will remain so until Scotland Yard is certain the threat has been contained.
MPs said they had heard three or four gunshots and staff inside Parliament were told to stay inside their offices.
Commons Leader David Lidington told MPs the "alleged assailant was shot by armed police".
Image copyright AP Image caption A man believed to be the suspect received medical treatment Shortly after the incident, a Downing Street source confirmed that Mrs May was safe.
The prime minister was seen being ushered into a silver Jaguar car as what sounded like gunfire rang out at Parliament during the incident.
The White House said later that Mrs May had spoken to President Donald Trump about the attack.
'Screams and commotion'
Parliament was put into lockdown shortly after the incident at about 14:40 GMT.
Tom Peck, political editor for the Independent, tweeted: "There was a loud bang. Screams. Commotion. Then the sound of gunshots. Armed police everywhere."
Media captionSteve Voake, eyewitness: "I tried to stop people coming on to the bridge" Press Association political editor Andrew Woodcock witnessed the scenes unfolding from his office window overlooking New Palace Yard.
"I heard shouts and screams from outside and looked out, and there was a group of maybe 40 or 50 people running round the corner from Bridge Street into Parliament Square.
"They appeared to be running away from something.
"As the group arrived at the Carriage Gates, where policemen are posted at the security entrance, a man suddenly ran out of the crowd and into the yard.
"He seemed to be holding up a long kitchen knife.
"I heard what sounded like shots - I think about three of them - and then the next thing I knew there were two people lying on the ground and others running to help them.
"Armed police were quickly on the scene and I heard them shouting to people to get out of the yard."
'Mowed down'
An eye witness, Radoslaw Sikorski, a senior fellow at Harvard's Centre for European Studies, posted a video to Twitter showing people lying injured in the road on Westminster Bridge.
Image copyright Alison Baskerville Image caption Parliament went into lockdown after the attack Image copyright AP Image copyright AP He wrote: "A car on Westminster Bridge has just mowed down at least 5 people."
Scotland Yard said it was called to a firearms incident on the bridge amid reports of several people injured.
Transport for London said Westminster underground station has been shut at the police's request, and buses diverted.
Media captionAerial views of Houses of Parliament Mr Lidington said: "It seems that a police officer has been stabbed, that the alleged assailant was shot by armed police.
"An air ambulance is currently attending the scene to remove the casualties.
"There are also reports of further violent incidents in the vicinity of the Palace of Westminster but I hope colleagues on all sides will appreciate that it'd be wrong of me to go into further details until we have confirmation from the police and from the House security authorities about what is going on."
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