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.
Police said the incident was being treated as related to terrorism.
BBC television footage showed the vehicle approaching the gate at a low speed. The hatchback was filmed heading towards the gates, slowing down, and then driving into the gate but still at a low speed. The vehicle came from a car park opposite Downing Street, witnesses said.
A Met police spokesperson: “At approximately 16:20hrs on Thursday, 25 May a car collided with the gate of Downing Street on Whitehall.
“Armed officers arrested a man at the scene on suspicion of criminal damage and dangerous driving. He has been taken into custody.
“There were no reports of any injuries.
“Enquiries are ongoing to establish the circumstances.”
Prime Minister Rishi Sunak’s office declined to comment.
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Footage from the scene show a silver car with the boot open at the gates of Downing Street with police urging crowds to get back.
The car was seen up against the wrought iron gates which are protected by armed officers.
A handcuffed man was marched away by police who searched the car.
Witness Simon Parry, 44, 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.”
He said the driver had his “face to the floor” as he was being arrested.
He added: “There were sniffer dogs and a bomb squad... We saw people that were in a panic running away and we saw people who were excited.”
Protests are regularly held at the gates and security is very tight.
There were cordons around Whitehall with police officers blocking access to the street from outside the Ministry of Defence.
A police cordon was initially put in place in the Whitehall area outside Downing Street, but was later removed.
A tow truck then removed the vehicle from the area.
Eyewitness, Krish Kandiah, who was delivering a petition at Downing Street when the car crashed into the gates, told Sky News that there was an “almighty smash at the front gate”.
He said: “Immediately we saw police scramble and armed officers responded swiftly. They shouted at the man to get out. He was wrestled out of the car and put down on the ground and handcuffed.”
The incident came as many civil servants were leaving their offices for the day in the area.
Shortly after the incident, a video posted online showed a convoy of cars leaving the back of Number 10.
Mr Sunak was working from Downing Street on Thursday afternoon as was Chancellor Jeremy Hunt. The Prime Minister had been planning to leave No 10 anyway and left in the convoy.
It is understood local Westminster police are investigating the incident and not counter-terror officers.
Gates were put up at the entrance to Downing Street in 1989 as a result of IRA bomb attacks in London.
In a statement released nearly two hours after the crash, the Metropolitan Police said: “A small cordon remains in place outside Downing Street after a car collided with the gates earlier this afternoon.
“The incident is being dealt with by local officers in Westminster and isn’t currently being treated as terror-related.”