Police arrested a man near London's Natural History Museum on Saturday after a vehicle apparently drove into pedestrians, injuring a number of people.
Crowds in the busy tourist spot in South Kensington, which is also home to the Victoria & Albert Museum and the Science Museum, fled screaming in panic, an AFP reporter said.
Armed police were at the scene, and a video posted on Twitter showed a man being held down on the road next to a black car that appeared to have crashed.
Security is high in Britain after five terror attacks in six months -- four of them in London and one in Manchester -- with the bloodshed claiming 35 lives.
London's Metropolitan Police said they were called to the scene at 2:21 pm (1321 GMT) after reports of a collision.
"It is believed that a number of pedestrians have been injured. A man (no further details) has been arrested at the scene," the police said in a statement.
"Enquiries to establish the circumstances and motive are under way."
A spokesman for the museum told AFP: "I can confirm that there has been a vehicle collision with pedestrians on our Exhibition Road entrance."
One witness, who gave his name only as Leonard, told AFP: "I was near South Ken station and there were many police.
"Suddenly a police woman officer shouted at the crowd to run, and everybody flew in panic, many people screaming."