A man accused of deliberately hitting pedestrians with a car before crashing outside the Houses of Parliament has been arrested on suspicion of terror offences.
Counterterror police are leading an investigation into the incident, which left two victims in hospital and sparked a large security operation and evacuations in Westminster.
The silver Ford Fiesta hit several cyclists and pedestrians before crashing into security barriers outside the Houses of Parliament at 7.37am.
Metropolitan Police Assistant Commissioner Neil Basu, the head of UK counterterror policing, 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...no other suspects at the scene have been identified or reported to the police.
"There is no intelligence at this time of further danger to Londoners or the rest of the UK connected to this incident."
Two white vehicles appeared to be behind the car shortly before the crash but Mr Basu said there "was not a police car in pursuit of the vehicle" and a nearby ambulance with its blue lights on was on a "completely unrelated call".
The driver, a man in his late 20s, was arrested at the scene by armed officers and has been taken to a south London police station where he remains in police custody.
Mr Basu said the police's current priority was to formally identify the suspect "and establish his motivation if we can - he is not currently cooperating".
He added: "On the details we have at the moment, we don't believe this person was known to either MI5 or counterterror policing".
Investigators said no one else was in the vehicle and no weapons have yet been recovered amid ongoing searches.
They are treating the crash as a terrorist incident but have not yet given any details of potential motivations, and witnesses did not hear the driver shout anything while being arrested.
According to open source data, the car used in the attack is a 2010 Ford Fiesta Zetec, which was first registered near Nottingham. It displayed a sticker with the website of a car dealership that had 11 branches across the north of England, in Yorkshire, Manchester, Cheshire and Merseyside.
Aerial footage showed the car driving around the side of Parliament Square before swerving across a central reservation on to the wrong side of the road, hitting cyclists waiting at traffic lights.
It then crossed over another reservation before hitting security barriers near an entrance to the House of Lords.
Barry Williams, a BBC employee who works near the scene of the crash said he heard “screams”.
“The car went onto the wrong side of the road to where cyclists were waiting at lights and ploughed into them,” he added.
“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.”
Jason Williams told the BBC Radio 4 Today programme the driver had “driven at speed - more than 40 mph”.
He added: “I saw at least 10 people lying down. I was told basically to move away, to run. I have run for my life…it looked deliberate... it didn't look like an accident”.
None of the pedestrians injured are in a life-threatening condition and the London Ambulance Service said it had taken two people to hospital.
Police said one man has been discharged and a woman remains in hospital undergoing treatment for serious injuries. A third victim, a man, was also treated at the scene.
Theresa May said her thoughts were with those injured and thanked the emergency services for their “immediate and courageous” response.
A meeting of the government's emergency Cobra emergency committee was to be held at 2pm.
Sadiq Khan, the mayor of London, said he had been in contact with the Metropolitan Police commissioner over the incident, adding: "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."
Sajid Javid, the home secretary, said: “Huge thanks to our emergency services for their rapid reaction to incident in Westminster this morning. My thoughts are with those injured.''
Police have launched a huge security operation seeing large parts of Westminster shut down, including the Tube station, and cordons spread up Whitehall and across Westminster Bridge.
Explosives detection dogs were seen carrying out checks with its handler as armed officers patrolled cordons and a police helicopter hovered overhead.
“Officers remain at the scene and cordons are in place to assist the investigation. Road closures in the area are likely to remain in place for some time. Westminster Tube station is currently closed,” a police spokesperson said.
“The public can also expect to see extra police officers - both armed and unarmed - out on the streets in and around central London.
“The public should remain vigilant and if you see anything suspicious that causes concern, dial 999 immediately.”
Police initially locked down the Houses of Parliament and surrounding buildings but later let parliamentary staff into work and tours were planned to restart for tourists as soon as possible.
MPs and peers are currently on summer recess, meaning Westminster Palace is less busy than normal.
A spokesperson for the Houses of Parliament said: "The safety and security of those working in or visiting the UK Parliament is our top priority. Our current measures are robust and are continuously reviewed."
Officers were redirecting commuters and tourists around cordons as people gathered to look at the scene.
Rebecca Clark, a 52-year-old American tourist, was due to go on a tour of the Houses of Parliament this morning.
She told The Independent: She said: “I'm just frustrated mainly - I live in New York so we have these kind of things as well. They are being cautious and that's good.”
Ms Clark said the incident had not affected her view of London, where she previously studied and lived in the 80s. “It's always a good time to visit London, this is just unfortunate.”
Police appeared to be taking full precautions following the Isis-inspired attack that saw a car ploughed into pedestrians on Westminster Bridge and gates outside parliament last March.
The atrocity caused parliamentary security to be increased, with strengthened fencing installed around the building, protocols changed and new security barriers to prevent vehicles approaching entrances.
Security services have since foiled several other plots to attack Westminster using knives and bombs.
Last month a former Taliban bombmaker was jailed for life after being arrested walking towards Parliament armed with three knives. Westminster was also the main target of Britain's first all-female jihadi gang, who have been jailed for planning an Isis-inspired knife
Westminster was among the attack locations listed by another Isis fanatic who was training an "army of children" to launch simultaneous attacks in London.
And another jihadi planned to bomb the gates to nearby Downing Street before storming the building in a suicide vest to behead the prime minister.
Isis has issued advice to its followers non how to carry out atrocities using vehicles, knives and other easily obtained items, which have made attack plans faster and harder to spot for authorities.
The terrorist group urges perpetrators to carry a secondary weapon inside cars to inflict more damage after they crash, as seen in the previous attacks in Westminster and London Bridge.
Police are urging anyone with information on the incident or footage to call 0800 789 321 or visit www.ukpoliceimageappeal.co.uk.
