A man has been arrested on suspicion of terrorism offences after a car crashed outside the Houses of Parliament, injuring several people.
Officers were seen surrounding the vehicle before a man in his late 20s was arrested, shortly after 7.30 in the morning local time.
Cyclists were hit in the crash and London Ambulance Service said it had taken two patients to hospital with non life-threatening injuries.
No one else was in the car and no weapons have been found so far.
"At this stage, we are treating this as a terrorist incident and the Met's Counter Terrorism Command is now leading the investigation," said Scotland Yard.
The man is in custody at a south London police station and the vehicle is being searched at the scene.
A number of eyewitnesses have said the silver Ford Fiesta, which was travelling westbound, appeared to deliberately hit members of the public as it swerved into the opposite lane.
Footage released by the BBC shows the moment when the vehicle drives over a pedestrian crossing before crashing into the security barriers.
Parliament is currently not sitting.
Images posted to social media showed a man in handcuffs being led away from the car by officers.
A London Ambulance Service spokesman said the two people taken to hospital had "injuries that are not believed to be serious" and a third patient with minor injuries was assessed at the scene.
Breaking: Big armed police response to car which has cashed into Parliament barriers we are now being moved back pic.twitter.com/rYAqExq6rn— Vincent McAviney (@VinnyMcAv) August 14, 2018
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?"
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."
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."
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.
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".