LONDON (Reuters) -A man was arrested on Thursday after a car collided with the front gates of Downing Street, site of British Prime Minister Rishi Sunak's office and residence, in an incident London police said was not currently being treated as "terror-related".

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.

"A small cordon remains in place outside Downing Street after a car collided with the gates earlier this afternoon," London police said in a statement.

"The incident is being dealt with by local officers in Westminster and isn't currently being treated as terror-related."

Police had said earlier armed officers had arrested a man at the scene on suspicion of criminal damage and dangerous driving. Enquiries were ongoing and there were no reports of injuries.

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

Sunak's office declined to comment.

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.

(Reporting by Andrew MacAskill, Sachin Ravikumar and Kylie MacLellan, writing by William James; editing by Kate Holton, Nick Macfie, Alexandra |Hudson)