Two people have been injured after a car crashed into security barriers outside the Houses of Parliament.
Officers were seen surrounding the vehicle before a man was arrested, shortly after 07:30 BST.
Cyclists were hit in the crash and London Ambulance Service said it had taken two patients to hospital with non life-threatening injuries.
Scotland Yard’s counter-terrorism unit is leading the investigation but the force is “keeping an open mind”.
Armed police, ambulances and firefighters are continuing to respond to the incident in central London.
While a number of eyewitnesses have said the car appeared to deliberately hit members of the public, police cannot say if the incident is related to terrorism at this stage.
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.
Mayor of London Sadiq Khan said he was in close contact with police.