Terrified witnesses have described watching a terrorist use a 4x4 as a weapon to send pedestrians flying like skittles - including one thrown into the Thames in London - before plunging two 20cm knives into a policeman.
Michael Adamou, 25, was driving the 453 bus when he said he saw the grey Hyundai 4x4 speeding in the direction of London's Houses of Parliament.
He said: "As I turned left out of Whitehall to go over the bridge, I saw a car knock two people over.
"Initially I thought 'he's lost control of the car' but then I see quite a few people rushing into Parliament and police trying to stop everyone.
"After a few seconds I heard gunshots. Everybody on my bus was panicking."
"The first thing I saw was the two people laying lifeless on the floor. They were on the pavement.
"He must have come down the whole side of the bridge on the pavement. I would say there were at least 15 or 20 people.
"When I was getting everybody off the bus, because they made us leave the bus on the bridge, I heard one guy come running behind me shouting his wife had jumped into the river to avoid getting knocked down."
In this image taken from video emergency personnel gather around a body on the southside of Westminster Bridge. Photo / AP
Witnesses said the aftermath was like a scene from a "Hollywood disaster movie".
The attacked killed four people and injured up to 40, including some with "catastrophic" injuries.
The killer driver aimed at a group of tourists taking selfies in front of Big Ben and, armed with two knives, stabbed an officer protecting Parliament in the head and neck.
The killer began his rampage on Westminster Bridge, hitting and injuring a number of members of the public, including three police officers.
Witness Rob Lyon, 34, from Rugby, who works in marketing, said: "I heard a wheel definitely hit a kerb and quite a large crunch noise, I looked up and saw a car clearly hitting people as it came towards me.
"A colleague I was with, I heard him shout but I just instinctively jumped off the pavement and I could see people getting hit. Then the car just carried on up the bridge and I just looked round me in shock.
"I just saw people being hit by a car at speed, it was harrowing. I just sort of went into shock mode.
"I just looked around me and couldn't really work out what had happened.
"I was numb. It felt like I was in the middle of a Hollywood movie, you see these things but suddenly I was in the middle of it.
"My first thought is I wanted to tell my wife I was okay and my children.
"Then I wanted to check a lady I was near and I gave her coat as a pillow and waited with her.
"I called the emergency services I think I was one of the first to do so and waited with her. I couldn't see the driver. I saw it come up onto the kerb and then just drive straight. It was so quick. It probably joined the pavement 10 or 15 metres in front of me."
This was the moment gunshots were fired outside the UK Parliament.
"I saw the car mounting the cycle lane and go into the fencing and then the next thing I could see was a cyclist, I think, or some pedestrian laying down and there was a lot of steam coming out of the car and an awful lot of noise and shouting."
Former Polish Foreign Minister Radoslow Sikorski, who is now a senior fellow with the Harvard Centre for European Studies, was in a taxi on Westminster Bridge.
He said: "I heard what I thought was just a collision then I looked through the window of the taxi and I saw someone down obviously in great distress and then I saw a second person down. Then I saw three more people down one of them bleeding profusely."
Witness Rick Longley described the moment the attacker, believed to be carrying two knives, began stabbing a policeman.
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 surged around the corner by the gates opposite Big Ben.
"A guy came past my right shoulder with a big knife and just started plunging it into the policeman."
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.
Steve Voake, 55, was walking across the Westminster Bridge towards South Bank when he saw the aftermath.
He said he saw at least two bodies lying on the road and one in the water.
"I saw a trainer lying in the road and when I looked more closely I saw that there were a couple of bodies the other side of the road," he told the Press Association.
"And when I looked over the side there was another body lying in the water with blood all around it."
The photographs and video footage, much of it too distressing to publish, revealed the devastating aftermath of what had long been feared but sadly predicted - a seeming terrorist attack on the political centre of the United Kingdom.
Richard Tice told Sky News he saw between eight and 10 bodies lying on the ground on Westminster Bridge. "The car drove along the pavement knocking people over," said Tice.
The car, a Hyundai i40 SUV, mowed down anyone in its way.
The car then drove on to the pavement under the shadow of Big Ben and crashed into the iron railings that surround parliament. The driver, described as an Asian-looking man in his 40s, jumped out of the vehicle and ran down the road and around the corner to the gates of parliament that allow entry and exit for the cars of dignitaries.
Emergency services staff provide medical attention on Westminster Bridge. Photo / AP
The man was wielding what has been described as either a large knife or a machete. He then stabbed a police officer before being shot.
Witnesses heard three to six shots. The attacker had managed to reach inside parliament's compound, maybe 45m from the entrance gate before being stopped.
Rick Longley told the Press Association he saw the man stab a policeman.
Fighting back tears, 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 surged around the corner by the gates opposite Big Ben.
"A guy came past my right shoulder with a big knife and started plunging it into the policeman.
"I have never seen anything like that. I just can't believe what I just saw."
Quentin Letts, the Daily Mail's parliamentary sketch writer, said he saw a man dressed in black attack a police officer outside Parliament before being shot two or three times as he tried to storm 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 told the BBC.
Image 1 of 16: People stand near a crashed car and an injured person lying on the ground, right, on Bridge Street near the Houses of Parliament in London. Photo / AP
"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 10m towards the entrance, he said.
He added: "As this attacker was running towards the entrance two plainclothed guys with guns shouted at him what sounded like a warning, he ignored it and they shot two or three times and he fell."
