Parliament car crash - as it happened: Police search three properties as focus falls on terror suspect after 'appalling incident'
UK's terror threat level remains 'severe', Theresa May says, meaning fresh attack is highly likely
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Your support makes all the difference.Counter-terror police searched three properties in the Midlands after a car crashed into security barriers outside the Houses of Parliament, injuring three people.
Three police forces were involved in raids in Birmingham and Nottingham on Tuesday afternoon following the arrest of a 29-year-old British man on suspicion of terror offences.
Armed officers had swarmed Westminster at about 7.40am on Tuesday, when a silver Ford Fiesta ploughed into cyclists and pedestrians outside parliament before being halted by security barriers.
Images posted to social media showed a man, wearing a black puffer jacket and surrounded by officers, being led away in handcuffs from the hatchback. Detectives said later on Tuesday they believed the car had been driven to London from Birmingham overnight.
The Fiesta was spotted in the Tottenham Court Road area at 1.25am, the Metropolitan Police said, where it remained until about 6am. It was then driven to Westminster.
“There was not a police car in pursuit of the vehicle” when it crashed, Neil Basu, the Met’s head of counter-terrorism, told reporters.
Three people were hurt in the crash, one seriously.
Theresa May said that terrorists would “never succeed” in dividing the UK, after Donald Trump blamed “crazy animals” for the attack in a tweet.
The crash was an “appalling incident”, the prime minister said, adding that the terror threat facing the country remained “severe”, meaning another attack was highly likely.
See below how we covered developments in this story
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The length of Whitehall has been completely shut off to traffic, with the cordon extended up to where the road meets Trafalgar Square.
Our home affairs correspondent, Lizzie Dearden, is reporting from the scene.
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: "There was smoke coming out of the car. I have seen people on the ground, lying on the road. I don't know if they have actually been hit by the vehicle or not.
"I saw at least 10 people lying down. I was told basically to move away, to run. I have run for my life."
He added that "it looked deliberate... it didn't look like an accident".
Police initially locked down the Houses of Parliament and surrounding buildings but are now letting parliamentary staff into work.
Officers were redirecting commuters and tourists around cordons as people gathered to look at the scene.
"My thoughts are with those injured in the incident in Westminster and my thanks to the emergency services for their immediate and courageous response," the prime minister, Theresa May, has said on Twitter.
The driver of the car which crashed into a number of cyclists and pedestrians this morning is in his 20s and was arrested on suspicion of terrorist offences, Scotland Yard said.
"At 07:37hrs on Tuesday 14 August, a silver Ford Fiesta collided with a number of cyclists and pedestrians, before crashing into barriers outside the Houses of Parliament.
The driver of the car, a man in his late 20s, was arrested at the scene by armed officers. He has been taken to a south London police station where he remains in police custody.
"He was arrested on suspicion of terrorist offences.
"There was nobody else in the vehicle, which remains at the scene and is being searched. No weapons have been recovered at this stage.
"A number of people were injured as a result of the collision and they have been taken to hospital.
"At this early stage none of the injuries appear to be life-threatening.
"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.
"At this stage, we are treating this as a terrorist incident and the Met's Counter Terrorism Command is now leading the investigation."
Cyclist James Maker, 30, of Chelmsford, Essex, passed the scene in Westminster within minutes of the crash to see a woman injured on the floor and the car crashed into the barrier.
"I looked to the right-hand side and there was a cyclist on the floor, clearly injured," he told the Press Association.
"It was a woman, they were clearly quite injured, they weren't moving and they were in the recovery position."
Fellow cyclists were helping her, he said, adding: "There were a couple of cyclists who had clearly been involved who were sitting on the floor and perhaps had minor injuries.
"I must be honest, I've got a young son and a wife and my initial reaction was as long as paramedics were seeing to the woman, I wanted to get away from the incident."
The head of policy at the County Councils Network then saw the silver Ford Fiesta crashed into the barrier outside Westminster Palace.
"You've got to have an intent to go for that barrier, there's so many security barriers and the way the car ended up in that part it would appear deliberate to me. It's gone up the driveway and hit the actual barrier thinking perhaps it would open," he said.
The home secretary, Sajid Javid, said: "Huge thanks to our emergency services for their rapid reaction to incident in Westminster this morning. My thoughts are with those injured."
Scotland Yard has issued an appeal for anyone with information which could assist in the investigation into the crash to come forward.
British Transport Police (BTP) said it would be putting extra officers on patrols in England, Scotland and Wales on Tuesday afternoon and into the evening following the Westminster terror attack.
Superintendent Chris Horton from BTP said: "We know incidents such as this are likely to cause concern, so our officers will be highly visible both on board trains and at stations.
"We are there to reassure the travelling public, so please don't be alarmed if you see our officers, including firearms officers, on your journey."