Four people have died and at least 20 others have been injured after a terror attack in Westminster.

A police officer who was stabbed at the Houses of Parliament and his attacker are among the dead, Metropolitan Police Acting Deputy Commissioner and Head of Counter Terrorism Mark Rowley has confirmed.

He told press outside Scotland Yard: "Sadly I can confirm that now four people have died and that included the police officer who was protecting Parliament and one man we believe to be the attacker who was shot by a police firearms officer.

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The aftermath outside Westminster

Paramedics fought to save the officer's life and that of his attacker on the floor of the cobbled courtyard in front of Parliament, with Foreign Office minister Tobias Ellwood among those who rushed to help.

The police officer was wheeled away on a stretcher with his face covered.

Mr Ellwood, who lost his brother in the Bali bombing, could be seen pumping the officer's chest then standing above him, his hands and face smeared with blood.

Other armed officers, some in plain clothes and wearing balaclavas, swarmed around the yard just feet from where MPs had earlier attended Prime Minister's Questions.

The knifeman drove a grey Hyundai i40 across Westminster Bridge before crashing it into railings then running through the gates of the Palace of Westminster.

His attack left a trail of destruction as paramedics tended to victims on the bridge and at the gate.

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Forensics at Westminster Bridge

One woman hit by the attacker's car before he reached Parliament was confirmed dead by a doctor at St Thomas' Hospital. She said others on the bridge suffered 'catastrophic injuries'.

Another woman who apparently fell into the Thames was rescued and given urgent medical treatment on a nearby pier.

London Ambulance Service said paramedics had treated at least 10 patients on Westminster Bridge.

A party of French schoolchildren were among those targeted on the bridge, with three injured.

Downing Street said Prime Minister Theresa May, who was ushered away from Parliament after the attack, was chairing a meeting of the Government's emergency Cobra committee.

Mrs May was seen being ushered into a silver Jaguar in the grounds of the palace as what sounded like gunfire rang out at around 2.45pm.

Eyewitness Rick Longley described the attack.

He said: "We were just walking up to the station and there was a loud bang and a guy, someone, crashed a car and took some pedestrians out.

"They were just laying there and then the whole crowd just surged around the corner by the gates just opposite Big Ben.

"A guy came past my right shoulder with a big knife and just started plunging it into the policeman.

"I have never seen anything like that. I just can't believe what I just saw."

An air ambulance landed in Parliament Square and a regular ambulance came in through the front gates as medics rushed to help.

Armed police cleared the area around the incident and Parliament Square and surrounding streets were closed to traffic while Parliament went into lockdown.

As the sitting in the House of Commons was suspended, Commons Leader David Lidington told MPs: "What I am able to say to the House is 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."

Witnesses described seeing a heavy-set man carrying a knife and running towards the Houses of Parliament.

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Westminster was in lockdown following the attack

Jayne Wilkinson said: "We were taking photos of Big Ben and we saw all the people running towards us, and then there was an Asian guy in about his 40s carrying a knife about seven or eight inches long.

"And then there were three shots fired, and then we crossed the road and looked over. The man was on the floor with blood.

"He had a lightweight jacket on, dark trousers and a shirt.

"He was running through those gates, towards Parliament, and the police were chasing him."

Frazer Clarke, 25, from Burton-on-Trent, one of a group of young boxers who witnessed the attack, said: "We heard a loud bang and screaming and then I noticed some smoke. I thought it was a car crash.

"I looked towards the front gate and people were running, a police officer and a fellow coming to the gate with two knives.

"He was stabbing the police officer with the knives.

"He was wearing black tracksuit bottoms, a black or grey top and what looked like work boots.

"The police officer was stumbling and fell on the floor.

"Another police officer walked toward the guy with the knives and shot him twice."

Another witness, from South Africa and living in Ireland, said she and two friends went towards the palace gates and saw a man lying on the floor inside.

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An air ambulance outside the Commons

She said: "I saw a body, they were working on the floor. They were just inside the gates.

"He had one gunshot wound to the right side of his chest. I don't know if he was dead or alive, but people were working on him.

"I saw that needed medical attention and I asked if I could help because I am a doctor, but they said they were fine."

In his statement Mr Rowley added: "As you know, we've declared this as a terrorist incident and the counter-terrorism command are carrying out a full-scale investigation into events today.

"A crime scene will remain in place in the affected areas in Westminster and you will understand it is vital as we carry out a painstaking investigation to recover all the possible evidence.

"This is a day we planned for but we hoped would never happen. Sadly it's now a reality.

"We will continue to do all we can to protect the people of London."