Armed officers descended on Whitehall after a car crashed into the gates of Downing Street.
The Met said a suspect is being held on suspicion of criminal damage and dangerous driving, but the incident is not being treated as terror related.
One witness said he saw officers pointing Tasers at a man, who was held “face to the floor” as he was detained.
No one was injured in the incident. No 10 has confirmed Rishi Sunak was in Downing Street at the time.
The area in Whitehall, the main road which runs through the heart of several government offices, was partially evacuated following the incident (Thursday) at 16:20 BST.
The road has since reopened to traffic but police tape remains in place at the scene and the car remains in place.
Officers have been seen searching the car, and sniffer dogs were also spotted at the scene.
Witness Simon Parry, 44, said he heard a “bang” and saw police pointing Tasers at a man.
“A lot of police vehicles came very quickly and were very quick to evacuate the area,” he told PA news agency.
He said the man appeared to slow down before he reached the gates of Downing Street, having driven from the other side of the road.
“There were sniffer dogs and a bomb squad,” he added.
Describing the mood on Whitehall in the aftermath of the incident, Mr Parry said: “We saw people that were in a panic running away and we saw people who were excited.”
Another witness, Matthew Torbitt, 32, said he heard a loud bang and was stopped on Whitehall after police locked down the area.
Footage of the car, identified as a silver Kia, shows it slowing down as it approaches the main entrance to Downing Street, the home of Prime Minister Rishi Sunak and his family.
The PM and the chancellor were in Downing Street at the time of the crash. Mr Sunak has since left for a scheduled visit.
The Downing Street gates are staffed around the clock by armed and unarmed police officers but are accessible by road via Whitehall.__BBC.com