A car crashed into the gates of Downing Street on Thursday, triggering a huge security operation from police who closed surrounding streets to the public.
Images on social media showed the silver Kia car near Downing Street, surrounded by emergency vehicles.
No injuries were reported and the driver was arrested.
“I heard a bang and looked up and saw loads of police with Taser guns shouting at the man,” said witness Simon Parry.
“A lot of police vehicles came very quickly and were very quick to evacuate the area.”
The crash prompted a rapid security response in one of London's most fortified areas, where security cameras cover every direction.
It is understood that Prime Minister Rishi Sunak was inside his office at 10 Downing Street, which is also his official home.
“Armed officers arrested a man at the scene on suspicion of criminal damage and dangerous driving,” police said.
“There were no reports of any injuries. Inquiries are continuing to establish the circumstances.”
Mr Parry said the driver had his "face to the floor" as he was being arrested.
He said the car appeared to slow down before reaching 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.
Police said the car hit the gates of Downing Street on Whitehall at about 4.20pm.
The site of the crash is also a popular tourist area with the House of Commons, Westminster Abbey and Trafalgar Square all a short walk away.
Sections of Whitehall were closed to the public and vehicle traffic, with pedestrians being turned away from the main thoroughfare around Downing Street.
In a statement released almost 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."
The gates were put up in 1989 in response to threats from Irish Republican Army militants. Public access to the street is restricted and the barriers are protected at all times by armed police officers.
