An emergency COBRA meeting is being convened this afternoon, following a terror attack outside Parliament this morning.
Armed police officers arrested the man, who drove a silver Ford Fiesta into the security barriers outside the Houses of Parliament shortly after 7:30am, on suspicion of terror offences. He is not believed to be known to police, and has been taken to a south London police station.
Two people were hospitalised during the attack, and one of them remains in hospital with serious but non-life threatening injuries. Others were treated at the scene, where extensive cordons are still in place and the majority of entrances to the Parliamentary estate have been shut.
Eye-witnesses suggested the car was driven at speed towards a group of pedestrians and cyclists resulting in a loud bang when it hit the barriers. Armed police swooped on the driver who was pulled from the car and taken away.
The Met's Counter Terrorism Command is leading the investigation.
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.
But he added: “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 and the investigation is being led by officers from the Counter Terrorism Command.
“Officers are searching the vehicle and no other weapons have been found at this time.
“At this early stage of the investigation, no other suspects at the scene have been identified or reported to police. There is no intelligence of further danger to Londoners or the rest of the UK in connection with this incident.
“Our priority is to formally identify the suspect and establish his motivations if we can. He is not currently co-operating.”
The COBRA meeting will be chaired by Madeleine Alessandri, deputy national security adviser, and attended by various officials rather than ministers, many of whom are out of the country during summer recess.
Prime Minister Theresa May, who is currently in Switzerland, tweeted: “My thoughts are with those injured in the incident in Westminster and my thanks to the emergency services for their immediate and courageous response.”
London mayor Sadiq Khan said: "All Londoners, like me, utterly condemn all acts of terrorism on our city.
“The response of Londoners today shows that we will never be cowed, intimidated or divided by any terrorist attack."
Home secretary Sajid Javid and a number of MPs tweeted their praise for the emergency services.
The attack comes just 17 months after Khalid Masood drove into pedestrians on Westminster Bridge, killing four, before stabbing PC Keith Palmer to death.