This is the moment police shot an armed attacker outside the Houses of Parliament in a major terror incident.
A crowd of terrified people can be seen running away from the scene, then three shots can clearly be heard.
Two armed police officers are then seen getting out of a car and entering the Palace of Westminster amid the confusion.
The assailant stabbed one police officer and was attempting to attack another with an eight-inch blade when he was shot by an officer.
Witness Aaron Tsang, a Taiwanese tourist, told Yahoo UK: ‘A man with a knife was shot by police, twice.
‘Police jumped on the guy and the scene was evacuated very quickly.
‘I saw a lot of ambulances and police cars arriving, and the scene has been pushed back right back to Horse Guards Parade.’
Here are the latest updates
Police say attack is linked to Islamic extremism
At least seven people have been arrested after overnight raids in London and Birmingham
Four people were killed in the attack and 40 injured, some seriously
Terrorist among those dead after being shot by police
Killed police officer identified as PC Keith Palmer
Seven others remain critically injured
Police have asked for the identity of the terrorist not to be revealed by the media
Parliament to meet again as normal this morning
Three French students among those seriously injured on the bridge
Police have cleared the surrounding area
The attacker, armed with two large knives, mowed down pedestrians with his car on Westminster Bridge, including schoolchildren, then rushed at the gates in front of the Houses of Parliament, stabbing the policeman before being shot dead by other officers.
Eyewitnesses described scenes of terror when gunfire rang out as the man approached a second officer.
Scotland Yard’s top anti-terror officer Mark Rowley said the two other fatalities were on the bridge when the terrorist struck. At least 20 other people were injured.
Mr Rowley told reporters outside New Scotland Yard, just yards from the scene of the attack: ” This was a day we’ve planned for but hoped would never happen. Sadly it is now a reality.”
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.
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.
“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.
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.
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.”