A man has been arrested on suspicion of terrorism offences after a car crashed outside the Houses of Parliament.
One woman is being treated in hospital for serious but not life-threatening injuries, Scotland Yard said. Two others were also injured.
The man in his late 20s was held after the crash, shortly after 07:30 BST. He is not believed to be known to police.
Counter-terrorism police said the suspect was not cooperating and has not been formally identified.
Scotland Yard's head of counter terrorism Neil Basu said there was "no intelligence at this time of further danger" to London or the UK as a whole.
The man is in custody at a south London police station and the vehicle is being searched. The government's Cobra emergency committee is meeting later.
No one else was in the car and no weapons have been found so far. The Met's Counter Terrorism Command is leading the investigation.
A number of eyewitnesses have said the silver Ford Fiesta, which was travelling westbound, appeared to deliberately hit cyclists and pedestrians as it swerved into the opposite lane.
Footage released by the BBC shows the moment when the car drives over a pedestrian crossing before crashing into the security barriers.
Parliament is currently not sitting.
Westminster tube station is closed and streets around Millbank, Parliament Square and Victoria Tower Gardens have been cordoned off. Nearby Strutton Ground has also been closed to the public.
People were moved further back from the area twice as police put up a Terrorism Act cordon.
Images posted to social media showed a man in handcuffs being led away from the car by officers after the crash.
A London Ambulance Service spokesman said two people were taken to hospital with "injuries that are not believed to be serious" and a third patient with minor injuries was assessed at the scene.
One of those taken to hospital, a man, has since been discharged.
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."