A man has been arrested on suspicion of terrorism offences after a car crashed outside the Houses of Parliament, injuring two people.
Officers were seen surrounding the vehicle before the man in his late 20s was detained, shortly after 07:30 BST.
Cyclists were hit in the crash and London Ambulance Service said it had taken the patients to hospital with non life-threatening injuries.
Scotland Yard’s counter-terrorism unit is leading the investigation.
Armed police, ambulances and firefighters are continuing to respond to the incident in central London.
A number of eyewitnesses have said the silver car appeared to deliberately hit members of the public.
BBC News home affairs correspondent June Kelly described the terror arrest as a “significant development”.
“The police will be looking at this man’s background, his identity – they’ll either know it or they will be working towards it,” she said. “They will be looking at his beliefs his associates, also his mental state.
“Also, crucially, is he on their radar – is he somebody who was known to them?”
Westminster tube station is closed and streets around Millbank, Parliament Square and Victoria Tower Gardens have been cordoned off.
People have been moved further back from the area twice as police put up a Terrorism Act cordon, officers at the scene said. Parliament is currently not sitting.
An eyewitness said the car was travelling westbound past the Houses of Parliament when it swerved into eastbound traffic.
Images posted to social media showed a man in handcuffs being led away from the car by officers after the crash.
Prime Minister Theresa May said: “My thoughts are with those injured in the incident in Westminster and my thanks to the emergency services for their immediate and courageous response.”
Home Secretary Sajid Javid also thanked emergency services for their quick response.
Mayor of London Sadiq Khan said he was in close contact with police.
‘I ran for my life’ – eyewitness accounts
Barry Williams, a BBC member of staff based at Millbank, said: “I heard lots of screams and turned round.
“The car went onto the wrong side of the road to where cyclists were waiting at lights and ploughed into them.
“Then it swerved back across the road and accelerated as fast as possible and hit the barrier at full pelt.
“It was a small silver car and he hit it at such speed the car actually lifted off the ground and bounced.
“Then the police just jumped. Two officers managed to leap over the security barriers and then the armed police vehicles all sped towards the scene.”
Jason Williams told the BBC Radio 4 Today programme the driver had “driven at speed – more than 40 mph”.
He added: “There was smoke coming out of the car. I have seen people on the ground, lying on the road. I don’t know if they have actually been hit by the vehicle or not.
“I saw at least 10 people lying down. I was told basically to move away, to run. I have run for my life.”
He said that “it looked deliberate… it didn’t look like an accident”.
More than 10 police vehicles and at least three ambulances were at the scene outside Parliament.
Firearms officers and at least two police sniffer dogs have been searching the area.
A bus driver who was driving past the scene at the time said he saw smoke coming out of the car.
Victor Ogbomo said: “I just stopped the bus. 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.”
Ewalina Ochab, who also saw the crash, said: “I think it looked intentional – the car drove at speed and towards the barriers.”
She said: “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.”
The vehicle did not appear to have a front registration plate when it crashed, she added.
A London Ambulance Service spokesman said: “We have treated two people at the scene for injuries that are not believed to be serious and have taken them to hospital.”
Westminster station is closed for entry and exit, Transport for London said, and is open only for interchange between lines.
Rush hour traffic is being diverted and workers have been told there may be a long wait before roads are reopened.
The Supreme Court has also been closed to the public.
The Houses of Parliament are surrounded with security barriers of steel and concrete. The measures were extended in the wake of the Westminster Bridge attack in March 2017 when Khalid Masood ploughed a car into crowds on Westminster Bridge, killing four people.
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