A number of 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.
Scotland Yard cannot say if the incident is related to terrorism at this stage but a number of eye witnesses have said the car appeared to deliberately hit members of the public.
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 eye witness has reported to the BBC that the driver was travelling westbound past the House of Parliament when the car swerved into eastbound traffic.
The witness said the driver appeared to have deliberately hit cyclists queuing at traffic lights to enter Parliament Square.
He said the car then swerved back across the road and hit the crash barrier next to the parking area outside the House of Lords.
'Action appeared deliberate'
Images posted to social media showed a man in handcuffs being led away from the car by officers.
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."
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."
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.