The crash happened just after 7:30 BST on Tuesday morning. One man has been arrested on suspicion of terror offences.
Several people were injured in the incident.
Westminster crash: Counter-terror police lead investigation – what we know so far
A man has been arrested after a car hit cyclists and pedestrians and crashed into security barriers outside parliament.
This is what we know so far:
:: The driver of the car, a man in his late 20s, was arrested at the scene by armed officers on suspicion of terrorist offences. He was wearing a black puffer jacket and was led away in handcuffs. He is being held at a south London police station.
:: Police said the suspect was “not currently co-operating”, and they had not formally identified him. He does not appear to be known to either MI5 or counter-terrorism policing. According to Sky sources, he is believed to be from the Midlands.
:: Assistant Commissioner Neil Basu, who leads counter-terrorism policing in the UK, said the incident “appears to be a deliberate act”.
:: The Metropolitan Police’s counter-terrorism command is leading the investigation. It is being treated as a “terrorist incident”.
:: Witnesses said the crash looked deliberate. The car is said to have been driven at speed, and to have lifted off the ground when it hit the barriers.
:: No other suspects at the scene have been identified or reported to police.
:: There is “no intelligence at this time of further danger to Londoners, or the rest of the UK, connected to this incident”, Mr Basu said.
:: Two people were taken to hospital, one of whom, a man, has since been discharged. A woman is being treated for serious but non life-threatening injuries at St Mary’s Hospital in Paddington. Another man, who was treated at the scene, did not need to go to hospital.
:: Scotland Yard said the car was “in collision with barriers outside the Houses of Parliament” at 7.37am.
:: The Ford Fiesta “collided with a number of cyclists and pedestrians”.
:: There was no one else in the vehicle, and officers have not found any weapons so far.
:: The area was put into lockdown, with streets around Parliament Square, Millbank and Victoria Tower Gardens cordoned off. The whole of Whitehall was also shut to traffic, with the cordon extending all the way up to Trafalgar Square.
:: At lunchtime, cordons along Victoria Embankment were lifted and Parliament Square and Whitehall reopened. But the immediate area around the incident was still taped off.
:: Witness Ewalina Ochab said: “I think it looked intentional – the car drove at speed and towards the barriers.
“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.”
:: Another witness, Jason Williams, said: “I saw a car going at high speed towards Parliament.
“It hit a bollard. It looked deliberate. It didn’t look like an accident. How do you do that by accident? It was a loud bang.”