A car collided overnight with the gates of Downing Street in central London, where the British prime minister's home and offices are located, setting off a rapid security response in one of London's most-fortified sites.
The Metropolitan Police force said a man was arrested at the scene on suspicion of criminal damage and dangerous driving.
There were no reports of injuries.
Prime Minister Rishi Sunak was in his office at the time.
Police lifted a cordon imposed in London's government district less than two hours after the collision took place.
So far counterterrorism officers have not been called in to investigate the crash.
Video footage posted on social media showed a silver hatchback car heading straight for the gates at low speed across Whitehall, the main thoroughfare in London's government district.
Footage shot soon after showed a car with its trunk open up against the tall metal gates. Several police officers minutely inspected the vehicle, removing items from the trunk and inside the car and placing them in evidence bags.
It was not immediately clear whether the crash was deliberate. Police said they were working to establish the circumstances.
"I heard a bang and looked up and saw loads of police with taser guns shouting at the man," said witness Simon Parry, 44.
"A lot of police vehicles came very quickly and were very quick to evacuate the area."
The BBC showed a photo of officers leading away a man with handcuffed hands behind his back.
Officers cordoned off a wide area after the crash but began to lift the barriers within half an hour, allowing people back into Whitehall, which normally teems with civil servants and tourists keen to see the nearby Houses of Parliament and other historic buildings.
Downing Street is a narrow street with a row of Georgian houses that includes the prime minister's official residence at No 10.
Public access to the street is restricted and the heavy steel gates are protected at all times by armed police officers.
Concrete bollards and metal crowd barriers also help keep threats at bay.
The gates were erected in 1989 in response to threats from Irish Republican Army militants.
In 1991 the IRA fired three mortars at the street, one of which exploded in the backyard of No 10 while Prime Minister John Major was leading a Cabinet meeting inside. Three police officers and a civil servant suffered minor injuries.
The area was targeted in 2017, when an extremist inspired by the Islamic State group killed four people with a vehicle on Westminster Bridge before stabbing a police officer to death outside Parliament.