Eyewitnesses to the attack at UK's Parliament describe what they saw when a man mowed down pedestrians and stabbed a police officer.
Five people died, including a policeman, a woman, and the assailant in the attack. Police said 40 people were injured.
See how the attack unfolded live: here
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"This man 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," said Daily Mail journalist Quentin Letts, who was working in an office overlooking New Palace Yard.
"And we could see the man in black moving his arm in a way that suggested he was either stabbing or striking the yellow jacketed policeman and one of the policemen then ran to get help which was very quick to come.
"And as this attacker was running towards the entrance used by MPs to go into the House of Commons, as he was running - he ran about I'd say 15 yards perhaps - two plain clothes guys with guns shouted at him what sounded like a warning, he ignored it and they shot two or three times and he fell."
A group of young boxers, leaving a press conference ahead of a World Series Boxing event when they came face to face with the alleged assailant.
"We heard a loud bang and screaming and then I noticed some smoke. I thought it was a car crash," Frazer Clarke, 25, from Burton-on-Trent, told the Press Association.
"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.
"The police officer was stumbling and fell on the floor. Another police officer walked toward the guy with the knives and shot him twice.
"Then people started shouting at us to get inside. Time stood still and I was shocked."
Steve Voake was on the bridge itself.
He said: "I was just walking across the bridge and suddenly a bus stopped and everybody started screaming and people came off the bus and they seemed very upset.
"And then I saw what appeared to be a trainer by the side of the road and then on the other side of the road there was a body and when I looked further up there was another body and then when I looked over the side of the bridge there appeared to be a body in the water as well."
Mr Voake said he tried to stop others coming onto the bridge.
Another witness, Ismael, was waiting at a traffic light near Westminster Bridge.
He said: "As I was waiting for the light to change the car sped up and passed me.
"That small shop after the traffic light ... that's the first victim hit ... second after a few metres, and the third and by then it clicked that something was going on.
"It [the car] continued ... by the end of the bridge - I don't know what's happening but there was a lot of people on the pavement and the car went straight."
One New Zealander, Wellingtonian Craig Spanhake, was between the Houses of Commons and the House of Lords as a tourist and was trapped behind a cordon for over 5 hours.
"We saw a few waves of police go through with their automatic weapons and they told us they would be getting us out of there as soon as they could," he said.
MP: 'I heard four shots'
Conservative MP Grant Shapps told BBC News: "I was walking through the Cloisters when the division bell rang. There were a lot of people in Portcullis House, where the offices are, and as we were streaming across to vote, we heard shouting and saw police officers, guns drawn, pointing towards the gate.
"Then I heard four shots in quick succession. Instantly, there were police officers by our sides saying 'get down,' then 'get back', and we went inside behind the palace walls."
Former education secretary Nicky Morgan told the Press Association: "I was walking from Portcullis House along the path of Old Palace Yard when suddenly shots rang out.
"It takes a moment to realise that is actually gunfire and at that point people were yelling: 'Get down, get back!'
"We have at the back of our mind all the time that something terrible can happen. The first thing is is that actually gunfire? Have you misheard? But the reaction of the officers around made it very clear this is a very serious situation."
George Eaton, the New Statesman's political editor, watched the incident unfold from the press gallery inside the Commons.
He told Sky News: "I saw a large crowd fleeing the attacker who appeared to be carrying a knife.
"He then entered the gates of Parliament and charged at officers. We now know that one of them was sadly stabbed. He was then very swiftly shot by armed police."
Michelle Langham was with one of a group of school children visiting the Houses of Parliament at the time.
"We were in the cafe when it happened. We saw a policeman down on the floor. We saw a lot of commotion. Everyone who was out on the street was told to get into the cafe. There were lots of people screaming," she said.
"We haven't been told anything about the casualties, everything we are getting is from social media. Inside we are not being told anything."
Financial Times journalist Henry Mance was locked down in the parliamentary complex along with some "thousands" of people who "just happened to be in parliament today because they work here or visiting".
Bradford Buck, from Connecticut in the US, saw the incident unfolding.
"Police cars just kept coming one after another after another, I've never seen such a quick response," he said.
"My wife and I came up from Westminster underground [station], we walked across to Parliament and there was a car crashed into the gate there.
"Police officers were running with machine guns, and there was a man down right next to the car."
Fearing for their safety, Mr Buck and his wife sheltered behind some concrete, before police moved them to safety.
New Zealand journalist in London, Josh White, told Morning Report nothing could have prepared him for such a shocking event.
Mr White police have increased their patrols across London and were tightly controlling the flow of information.
There had been less "rhetoric" on social media than might have been expected, he said.
"People have actually been very careful to wait and see what the information is, and as the Met police said themselves, they won't be getting into anything they can't confirm yet."
- BBC / RNZ