A man has been arrested on suspicion of criminal damage and dangerous driving after a vehicle was driven into the gates of Downing Street.
The silver Kia car crashed into the gates at around 4.20pm on Thursday, the Metropolitan Police said in a statement.
Images on social media showed the car surrounded by emergency vehicles.
The force added that no-one has been hurt.
It is understood that counter-terrorism police are not involved in the investigation at this stage.
Rishi Sunak was in Downing Street at the time the car crashed into the front gates, the PA news agency understands.
The Prime Minister, who had been due to leave No 10 anyway, departed after the crash.
Witness Simon Parry, 44, said he heard a “bang” and saw police pointing Taser guns at a man.
“I heard a bang and looked up and saw loads of police with Taser guns shouting at the man,” he said.
“A lot of police vehicles came very quickly and were very quick to evacuate the area.”
Mr Parry said the driver had his “face to the floor” as he was being arrested.
He said the man appeared to slow down before he reached the gates of Downing Street, having driven out from the other side of the road.
“There were sniffer dogs and a bomb squad,” he added.
Asked to describe the mood in the immediate aftermath, Mr Parry said: “We saw people that were in a panic running away and we saw people who were excited.”
In a statement released on Twitter, the Metropolitan Police said: “At around 16:20hrs a car collided with the gates of Downing Street on Whitehall.
“Armed officers arrested a man at the scene on suspicion of criminal damage and dangerous driving.
“There are no reports of any injuries. Inquiries are ongoing.”
Footage of the incident broadcast by the BBC appears to show the car slowing down before it hits the gate.
A police cordon was initially put in place in the Whitehall area outside Downing Street, but was later removed.