A car collided on Thursday with the front gates of Downing Street, where British Prime Minister Rishi Sunak's office and residence is based, London police said, adding one person had been arrested on suspicion of dangerous driving.

BBC television footage showed the vehicle crossing the road towards Downing Street and approaching the gate at a low speed. The main gate appeared to be undamaged.

A witness who did not provide their name said the driver was a white man estimated to be in his 50s.

"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," police said in a statement. "There are no reports of any injuries."

Police said enquiries were ongoing.

A witness said there was a small vehicle outside the gate, with its doors and trunk open.

Sunak's office declined to comment.

Police put in place a small cordon blocking traffic along Whitehall, a main road running past Downing Street where several government departments are based.

Less than an hour later, police were examining the car as workers milled around a short distance away.

The current terrorism threat level in Britain is "substantial", meaning an attack is considered likely.

"Lots of children ran away and the police began pointing their guns at the person inside,” said Simon Parry, 44, who was protesting opposite Downing Street and saw the incident. (Reuters)