Police detained a man near London's Natural History Museum on Saturday after a vehicle apparently drove into pedestrians at a busy tourist spot, injuring a number of people.
Witnesses and footage posted on social media suggested the man was pinned down on the ground by passers-by near the Victoria and Albert Museum.
A police spokesman said it was too soon to say whether the incident was related to terrorism, and London Mayor Sadiq Khan said "details are still emerging".
The area, which includes the wide, semi-pedestrianised Exhibition Road which runs past the Science Museum, is popular with families and tourists, particularly on a Saturday afternoon.
Footage and photographs posted on Twitter showed the detained man next to a black car with its door open and its bonnet severely damaged.
Witnesses reported by some media said they had heard a loud bang.
An AFP reporter said crowds in the area had fled screaming in panic.
Armed police were deployed to the scene and the whole area was subsequently cordoned off. The public were urged to avoid the area.
Security is high in Britain after five terror attacks since March -- four of them in London and one in Manchester -- with the bloodshed claiming 35 lives.
Three of the attacks in London involved the use of a vehicle to plough into pedestrians.
Downing Street said Prime Minister Theresa May was being kept up to date with developments.
In a statement, Khan made no mention of terrorism but said he was in close contact with Mark Rowley, the head of counter-terrorism policing, and the emergency services.
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 later said the man had not been formally arrested, but had been detained.
One witness, identified only as Katy, told LBC London radio she had seen two people injured, including a young boy who had hurt his leg and a woman who was not moving.
Another 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."
A spokesman for the Natural History Museum told AFP: "I can confirm that there has been a vehicle collision with pedestrians on our Exhibition Road entrance."
Not far away, tens of thousands of football fans took part in a silent protest "against extremism", walking from Park Lane past Downing Street and to parliament.
The Football Lads Alliance was formed after the London Bridge attack in June and brought together supporters of rival teams with military veterans.