The scene has been locked down. Photo provide to chinadaily.com.cn

May was scheduled to chair a meeting of the government emergency committee, also known as known as COBRA, to discuss the response to the incident.

COBRA coordinates the high-level response to serious incidents. It brings together government ministers with senior officials of the emergency services and security and intelligence agencies.

David Lidington, Leader of the House Commons, told MPs at the time that police had shot an alleged assailant close to their chamber.

He told them: "What I am able to say to the House is that there has been a serious incident within the estate. 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."

The sitting of the House of Commons was suspended and MPs were told to stay in the chamber. All other government buildings were placed under "lock down". Police evacuated the House and other nearby buildings.

The Scottish Parliament in Edinburgh also suspended its debate on a second referendum being held on Scottish independence.

Witnesses described how the car plowed into pedestrians on Westminster Bridge before crashing into railings.

Rados?aw Sikorski, a Polish politician, told the BBC he saw four people on the ground on the bridge.

"I didn't see a car, but I heard what I thought was a collision," he said. "I looked through the window of the taxi and I saw someone down, obviously in great distress."

Bernard Cazeneuve, the French Prime Minister confirmed that French citizens, believed to be three school children, were injured in the car attack.

Amine Morada, from Algeria, was in Parliament Square when the incident happened: "I heard a loud bang, bang. I looked around and saw a man on the ground, with blood coming out of his head. He was white with black hair and was wearing a backpack. People were screaming. It was chaos. I saw a crashed car and two people were slumped on it. It had smoke coming out of it and water coming out the bottom. I thought it was an accident."

Quentin Letts, the Daily Mail's parliamentary sketchwriter, told the BBC he saw a man in black clothing attack a police officer, before being shot two or three times as he apparently tried to enter the House of Commons.

"I saw a thick-set man in black clothes come through the gates into New Palace Yard, just below Big Ben," he said. "He had something in his hand, it looked like a stick of some sort, and he was challenged by a couple of policemen in yellow jackets. And one of the yellow-jacketed policemen fell down and we could see the man in black moving his arm in a way that suggested he was stabbing or striking the yellow-jacketed policeman.

"The other officer ran to get help and the man in black ran about 15 yards towards the entrance.

"As this attacker was running towards the entrance, two plain-clothed guys with guns shouted at him what sounded like a warning, he ignored it and they shot two or three times and he fell."

Angus McNeice and Bo Leung contributed reporting from the scene.

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