Witnesses have described a car ploughing through cyclists and into a security barrier outside the Houses of Parliament in a "deliberate" attack, as police arrested a terror suspect.

Counter-terror police were leading the investigation after a Ford Fiesta crashed into "a number of" cyclists and pedestrians at around 7.40am on Tuesday, officers said.

Scotland Yard said the driver, a man in his twenties, was arrested at the scene by armed officers on suspicion of terrorist offences.

Geoffrey Woodman, a strategy consultant from Battersea, south-west London, had stopped at the traffic lights by Parliament Square on his cycle to work when the car swerved into cyclists and towards the security barriers.

"I was getting off the bike and put my foot down, then there was a sound like tyres screeching," he said.

"This car turned round to the left and swerved into the wrong lane of traffic and into the bank where all the cyclists wait."

He said most people managed to jump off their bikes to safety but one woman who "seemed in some distress" was clipped by the bonnet of the car as it passed.

"It went through everyone and turned round, continuing down the next 50 metres before turning into the lay-by where taxis pick up politicians, smashing into the barrier," he said.

Mr Woodman added that paramedics from a passing ambulance, captured on camera passing behind the car, stopped to help the cyclists and armed police arrived on the scene "within 10 seconds".

Ewalina Ochab, who asked to be identified only by her name, told the Press Association: "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, 45, from Kennington, south London, was walking to work when he saw the crash and heard a "loud bang".

Bus driver Victor Ogbomo, 49, 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 told the Press Association.

"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, 30, 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 told the Press Association.

"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.

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.