A man has been arrested on suspicion of criminal damage and dangerous driving after a car was driven into the gates of Downing Street.
The silver Kia car crashed into the gates at around 4.20pm on Thursday.
Images on social media showed the car surrounded by emergency vehicles and a man being led away.
Police said the incident is not being treated as terror-related.
Video footage broadcast by the BBC showed a car driving from the far side of Whitehall towards the gates, and appearing to slow down before impact.
Passers-by could be seen starting to run from the scene seconds later.
Photos showed a middle-aged white man being taken away by armed officers as police vehicles swarmed the area.
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The Metropolitan Police said a man had been arrested on suspicion of criminal damage and dangerous driving, and remained in custody as investigators established the circumstances of the incident.
“The incident is being dealt with by local officers in Westminster and isn’t currently being treated as terror-related,” a spokesperson added.
Rishi Sunak was inside 10 Downing Street when the crash happened, and a fleet of vehicles were seen leaving out of its back entrance shortly afterwards.
A spokesperson said the prime minister was already scheduled to leave but was slightly delayed by the crash and security scare.
Witness Simon Parry said: “I heard a bang and looked up and saw loads of police with Taser guns shouting at the man.
“A lot of police vehicles came very quickly and were very quick to evacuate the area.”
Mr Parry said the car appeared to slow down before reaching the gates of Downing Street, having driven out from the other side of the road, and the driver was arrested with his “face to the floor”.
“There were sniffer dogs and a bomb squad,” he added. “We saw people that were in a panic running away and we saw people who were excited.”
Police cordoned off Whitehall in the immediate aftermath of the crash and The Independent understands government staff in the area were asked not to leave buildings. Restrictions were later relaxed after security checks.
The Metropolitan Police said a car hit the gates that separate Downing Street, which houses the prime minister’s official residence, from Whitehall at around 4.20pm.
“Armed officers arrested a man at the scene on suspicion of criminal damage and dangerous driving,” a spokesperson added. “He has been taken into custody. There were no reports of any injuries.
“Enquiries are ongoing to establish the circumstances.”
Police were seen searching the car as the operation continued, and large sections of Whitehall were temporarily closed to the public and vehicles.