Police are searching three addresses after a suspected terror attack outside the Houses of Parliament.
A 29-year-old man, who is a UK national, was arrested on suspicion of preparing an act of terror after the silver Ford Fiesta he was driving collided with cyclists and pedestrians before crashing into a security barrier just before 7.40am on Tuesday.
Detectives believe the privately-owned car was driven from Birmingham late on Monday night, arriving in London just after midnight.
Scotland Yard said the vehicle was driven around the Westminster area from around 6am - more than an hour-and-a-half before the crash - having been in the Tottenham Court Road area between 1.25am to 5.55am.
Counter-terrorism officers are carrying out searches at two addresses in Birmingham and a residential property in Nottingham as part of the probe.
Metropolitan Police counter-terrorism head Neil Basu said the suspect, who is being held in custody at a south London police station, is not co-operating.
“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,” he told reporters.
“We have not formally identified him yet. On the details we have at the moment, we don’t believe this individual was known to MI5 or counter-terror police.”
The silver car can be seen driving along the road next to Parliament Square before moving to turn right towards Westminster Abbey in footage of the incident aired on BBC News.
As an ambulance passes the car on its right-hand side, the vehicle swerves left, crossing oncoming traffic and colliding with cyclists before entering a small road and crashing into a security barrier.
A police officer can be seen jumping another barrier that runs along the side of the road to get away.
Images posted to social media showed a man wearing a black puffer jacket being led away in handcuffs from the car as armed police officers swarmed the scene.
There was nobody else in the vehicle and no weapons were found, police said.
Mr Basu added no other suspects have been identified and there is “no intelligence at this time of further danger” to Londoners.
Two people were taken to hospital, while a third person with minor injuries was assessed at the scene, the London Ambulance Service (LAS) said.
One was discharged from St Thomas’ Hospital before midday, while a woman was being treated for serious but not life-threatening injuries at St Mary’s Hospital.
After a meeting of the Government’s emergency Cobra committee, Theresa May urged the country to come together and carry on as normal.
In a statement released by Downing Street, the Prime Minister praised the “formidable courage” and professionalism of the emergency services who “ran towards” danger.
She said: “The threat to the United Kingdom from terrorism remains severe.
“I would urge the public to remain vigilant but also to come together and carry on as normal, just as they did after the sickening attacks in Manchester and London last year.
“The twisted aim of the extremists is to use violence and terror to divide us. They will never succeed.”
Reacting to the suspected attack, US President Donald Trump said on Twitter: “These animals are crazy and must be dealt with through toughness and strength!”
Witnesses described an emotionless driver ploughing through cyclists in what appeared to be a deliberate act.
Kirsty Moseley, of Brixton, south London, was a passenger in the first car behind the cyclists, who “were thrown everywhere” after being struck at what she estimated was 25mph.
“I heard a few shouts, looked up and this silver car was driving at high speed the wrong way into the cyclists,” the 31-year-old told the Press Association.
“It’s absolutely amazing there was only one cyclist seriously injured, he went straight through them. People were thrown everywhere.”
Ms Moseley, who works in marketing and spoke to police as a witness, continued: “(He had) two hands on the steering wheel and he did not look back over his shoulder to look at the damage he’d created - he was just looking deadpan straight in front of him,” she said.
“He wasn’t shouting anything, he wasn’t screaming, he didn’t look crazed or out of control - he was just deadpan.”
Ewelina Ochab described it as an “intentional” act, while cyclist Geoffrey Woodman said he first heard a tyre-like “screeching”.
Mr Woodman, a strategy consultant from Battersea, added: “This car turned round to the left and swerved into the wrong lane of traffic and into the bank where all the cyclists wait.”
Jason Williams, from Kennington, was walking to work when he saw the “deliberate” crash.
“It didn’t look like an accident. How do you do that by accident? It was a loud bang,” the 45-year-old told PA.
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.
Masood abandoned his car then stabbed and killed unarmed Pc Keith Palmer before he was shot by armed police in a courtyard outside Parliament.
The terrorist threat against the UK is seen as unprecedented.
The Prime Minister’s official spokesman said there were 676 live investigations being carried out by the security services and counter-terror police at the end of June, up from more than 500 in March.
Some 13 Islamist plots and four by far-right extremists have been foiled in the past 18 months, he added.
There are roughly 3,000 active “subjects of interest” at any one time - while there is also a wider pool of more than 20,000 individuals who have previously featured in probes whose threat must be kept under review.