A man has been arrested by British authorities on suspicion of terrorism offenses after a car crashed outside the Houses of Parliament on Tuesday.
The vehicle swerved into cyclists and pedestrians shortly after 07:30 BST, injuring three people.
The suspect, in his late 20s, is not believed to be known to MI5 (Secret Intelligence Service) or counter-terrorism police and is not co-operating with officers.
Some eyewitnesses have said the silver Ford Fiesta, which was traveling westbound, appeared to deliberately hit members of the public as it swerved into the opposite lane.
One woman is being treated in hospital for serious but non-life-threatening injuries.
Scotland Yard's head of counter-terrorism Neil Basu said: "Given that this appears to be a deliberate act, the method and this being an iconic site, we are treating it as a terrorist incident."
He added there was "no intelligence at this time of further danger" to London or the UK as a whole.
The suspect, who has not been formally identified, is being held at a south London police station and the vehicle is being searched.
No-one else was in the car, and no weapons have been found. The government's Cobra emergency committee is meeting later.
Footage released by the BBC shows the moment when the car drives over a pedestrian crossing before crashing into the security barriers. A police officer can be seen jumping over another barrier to getting out of the way.
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.
A London Ambulance Service spokesman said two people had been taken to hospital with "injuries that are not believed to be serious," and a third patient with minor injuries had been assessed at the scene.
One of those taken to the 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," she said. "They will be looking at his beliefs, his associates, also his mental state."
From her side, 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 expressed his gratitude.
Source: International Islamic News Agency