Several people have been injured after a car crashed into security barriers outside the Houses of Parliament, Scotland Yard say.
Armed police, ambulances and firefighters are responding to the incident in central London, which happened shortly after 07:30 BST.
Officers were seen surrounding the vehicle before a man was arrested.
Reports say cyclists were hit but police do not believe anybody is in a life-threatening condition.
London Ambulance Service said two people were treated at the scene and have been taken to hospital.
Scotland Yard cannot say if the incident is related to terrorism at this stage but a number of eyewitnesses have said the car appeared to deliberately hit members of the public.
The Met Police said: “While we are keeping an open mind, the Met’s Counter-Terrorism Command is leading the investigation into the Westminster incident.”
Westminster tube station is closed and streets around Millbank, Parliament Square and Victoria Tower Gardens have been cordoned off.
People have been moved further back from the area as police put up a Terrorism Act cordon, officers at the scene said. Parliament is currently not sitting.
An eyewitness said the car was travelling westbound past the Houses of Parliament when it swerved into eastbound traffic.
Images posted to social media showed a man in handcuffs being led away from the car by officers after the crash.
‘I ran for my life’ – eyewitness accounts
Barry Williams, a BBC member of staff based at Millbank, said: “I heard lots of screams and turned round.
“The car went onto the wrong side of the road to where cyclists were waiting at lights and ploughed into them.
“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 said that “it looked deliberate… it didn’t look like an accident”.
The BBC’s Callum May, at the scene, said: “Parliament Square is now ringed by blue police tape. There is a large group of armed officers in the square.
“Paramedics are treating a small group of people sitting on the ground outside the Palace of Westminster.
“They are wrapped in silver blankets. One eyewitness, who declined to give his name, said he saw a small hatchback drive into a group of cyclists and then into the barrier outside the Palace of Westminster. The man said that the driver’s action appeared deliberate.”
More than 10 police vehicles and at least three ambulances were at the scene outside Parliament. Firearms officers and at least two police dogs are stationed inside Parliament Square.
A bus driver who was driving past the scene at the time said he saw smoke coming out of the car.
Victor Ogbomo said: “I just stopped the bus. The police said we have to move back, then in less than five minutes the response team came.
“They went to the vehicle, so we had to push back. I saw the car in the barrier, I didn’t know how it got there.”
Ewalina Ochab, who also saw the crash, said: “I think it looked intentional – the car drove at speed and towards the barriers.”
She said: “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.”
The vehicle did not appear to have a front registration plate when it crashed, she added.
Scotland Yard said: “At 07:37hrs today, a car was in collision with barriers outside the Houses of Parliament.
“The male driver of the car was detained by officers at the scene. A number of pedestrians have been injured. Officers remain at the scene. We will issue further info when we have it.”
London Ambulance Service said that as well as three ambulance crews, responders in cars were also sent to the scene.
“We have treated two people at the scene for injuries that are not believed to be serious and have taken them to hospital,” a spokesman said.
Westminster station is closed for entry and exit, Transport for London said, and is open only for interchange between lines.
Rush hour traffic is being diverted and workers have been told there may be a long wait before roads are reopened.
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.
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Westminster car crash: Man arrested as pedestrians injured}