Here is how witnesses describe what they saw as it unfolded:
Ewalina Ochab, who asked to be identified only by her name, said she thought the crash "looked intentional".
"The car drove at speed and towards the barriers."
She continued: “I was walking on the other side [of the road]. I heard some noise and someone screamed.
“I turned around and I saw a silver car driving very fast close to the railings, maybe even on the pavement.”
Jason Williams, from Kennington, south London, was walking to work when he saw the crash and heard a “loud bang”.
“I saw a car going at high speed towards Parliament. It hit a bollard,” he said.
“It looked deliberate. It didn’t look like an accident. How do you do that by accident?”
Bus driver Victor Ogbomo, was driving passengers past the front of Westminster just after the crash.
“All I saw was the smoke coming out of a vehicle, a silver vehicle… I just stopped the bus,” he said.
“The police said we have to move back, then in less than five minutes the response team came.
“They went to the vehicle, so we had to push back. I saw the car in the barrier, I didn’t know how it got there.
“I think someone was inside the vehicle because many police went towards the vehicle.”
He said officers had their guns aimed when they arrested the driver.
Cyclist James Maker, of Chelmsford, Essex, passed the scene in Westminster within minutes of the crash to see a woman injured on the floor and the car crashed into the barrier.
“I looked to the right-hand side and there was a cyclist on the floor, clearly been injured,” the head of policy at the County Councils Network said.
“It was a woman, they were clearly quite injured, they weren’t moving and they were in the recovery position."
Fellow cyclists were helping her, he said, adding: “There were a couple of cyclists who had clearly been involved who were sitting on the floor and perhaps had minor injuries.
“I must be honest, I’ve got a young son and a wife and my initial reaction was as long as paramedics were seeing to the woman I wanted to get away from the incident.”
Mr Maker then saw the silver Ford Fiesta crashed into the barrier outside Westminster Palace.
“You’ve got to have an intent to go for that barrier, there’s so many security barriers and the way the car ended up in that part it would appear deliberate to me. It’s gone up the driveway and hit the actual barrier thinking perhaps it would open,” he said.
He believes the car, driving clockwise around Parliament Square, then took left onto the exit on the wrong side of the road to get near the injured cyclist before crossing the road to the barrier.