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.
A woman is being treated in hospital for serious but non life-threatening injuries.
Scotland Yard’s head of counter terrorism Assistant Commissioner 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.
No-one else was in the car and no weapons have been found.
The government held a meeting of its Cobra emergency committee and the prime minister has paid tribute to the “formidable courage” and professionalism of the emergency services who “ran towards a dangerous situation in order to protect the public”.
Urging the public to remain vigilant but “carry on as normal”, Theresa May added: “For the second time in as many years the home of our democracy, which is a potent symbol of our precious values of tolerance and freedom, has witnessed terrible scenes just yards from its door.”
The suspect, who has not been formally identified, is being held at a south London police station and the vehicle is being searched.
BBC home affairs correspondent Daniel Sandford said sources have told him the suspect is from the Birmingham area.
A number of eyewitnesses have said the silver Ford Fiesta, which was travelling westbound in Parliament Square, 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 then crosses over a traffic island in the middle of the road 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 was closed for entry and exit, and streets around Millbank and Parliament Square cordoned off for several hours after the crash.
The immediate area around the incident remains taped off and screened by white temporary fencing.
Parliament is not currently sitting.
Two people had been taken to hospital, while a third person with minor injuries was assessed at the scene.
One of those taken to hospital, a man, has since been discharged.
‘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 on to 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.
“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.”
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 saw at least 10 people lying down. I was told basically to move away, to run. I have run for my life.”
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.”
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”.