Politicians, journalists, tourists and members of the public became witnesses to a terror attack in the heart of London, which left four people dead and at least 20 people injured.
An armed police officer and the attacker were among those who died during the incident outside the Houses of Parliament.
It began at 14:40 GMT on Westminster Bridge, crossing the River Thames between the South Bank and Parliament Square.
Michael Adamou, 25, was driving the 453 bus when he saw a grey Hyundai heading towards the Houses of Parliament and hitting pedestrians on the bridge.
"As I turned left out of Whitehall, so turning left to go over the bridge, I saw a car knock two people over," he said.
"Initially I thought he's lost control of the car, but then I saw quite a few people rushing into parliament and police trying to stop everyone.
"After a few seconds I just heard gunshots. Everybody on my bus was panicking."
He added: "I did not see him coming over the bridge but I saw everybody laying on the bridge, not moving."
One 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."
'Everybody started screaming'
Steve Voake was another person who witnessed the attack on the bridge.
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.
"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."
Richard Tice was coming out of the Tube station as the events unfolded.
He told BBC News: "I came out of Westminster Underground and it was clear something dramatic had happened.
"I was ushered on to the bridge and, looking up and down, there were at least eight figures on the west pavement… and then the car crashed at the south end of the bridge."
Bradford Buck, from Connecticut in the US, saw the incident alongside his wife.
"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, 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.
But the incident on the bridge was not the end of the attack as an assailant made his way into the grounds of the House of Parliament.
"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 were also caught up in the incident.
They were leaving a press conference ahead of a World Series Boxing event when they came face to face with the alleged attacker.
"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."
Politicians on their way to vote witnessed the chaos.
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."
Politicians, journalists and visitors were all shut inside the Houses of Parliament for hours as police carried out their investigation.
Michelle Langan was visiting the Houses of Parliament at the time.
She was shut in the central lobby after the incident and said lots of schoolchildren were there.
"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."
Around five hours later, people were allowed out of buildings to discover what had happened on the outside.