A man has been arrested on suspicion of terrorism offences after a car crashed outside the Houses of Parliament.
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 or counter-terrorism police, and is not co-operating with officers.
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.
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 car drives over a pedestrian crossing before crashing into the security barriers. A police officer can be seen jumping over another barrier to get out of the way.
Westminster tube station is closed for entry and exit, and streets around Millbank and Parliament Square have been cordoned off.
Parliament is not currently sitting.
A London Ambulance Service spokesman said two people had been taken to hospital with “injuries that are not believed to be serious”, while 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,” she said. “They will be looking at his beliefs, his associates, also his mental state.”
‘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.”
Another witness, called Kirsty, said: “A car drove the wrong way round the road, drove through about 20 cyclists and crashed into a wall. It looks like only one woman was seriously injured.”
Jason Williams told BBC Radio 4’s 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 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 – where firearms officers and police sniffer dogs have been searching the area.
British Transport Police said it was increasing patrols in England, Scotland and Wales and that its officers would be “highly visible on trains and at stations”.
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.”
By Dominic Casciani, BBC News home affairs correspondent
Security arrangements at Parliament have progressively tightened since 2001. In the aftermath of the 7/7 attacks, new truck bomb-proof barriers were installed in an effort to “target harden” Westminster.
These include the reinforced low black rampart-like walls that surround Parliament itself and a highly visible armed police presence.
Visitors need to go through a chicane-like system designed to help armed officers spot suspects. The one significant weak spot was the main vehicle gates – as became apparent in March 2017’s one-man attack.
That triggered an internal security review that has led to changes that remain secret – although it’s apparent to Londoners that there are now more armed police than ever before patrolling the area.
The threat of terrorism is a constant concern for Parliamentarians – it’s not new. The IRA murdered Airey Neave MP in 1979 in a car bomb that exploded within the palace grounds. And the dilemma remains the same: how best to balance security with guaranteeing that the heart of British democracy remains open to the people.
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.
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.
Mayor of London Sadiq Khan said he was in close contact with police and that he “utterly condemns all acts of terrorism on our city”.
And Labour leader Jeremy Corbyn said the bravery of emergency services “keeps us safe day in, day out”.
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.
Scotland Yard is asking anyone with information on Tuesday’s crash to call 0800 789 321. Anyone with video or pictures of the crash is asked to send them to police.
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Westminster car crash: Man arrested on suspicion of terror offences}