Almost a dozen people were injured after a car mounted the pavement outside London’s Natural History Museum.
Video footage posted on Twitter showed a man being restrained on the ground by members of the public after the incident in South Kensington.
The incident sparked fears of a terrorist attack in an area busy with families, but Scotland Yard later confirmed it was being treated as a road traffic collision and was "not being treated as a terror-related incident".
Eleven people treated were mostly suffering from head or leg injuries, the London Ambulance Service said, with nine taken to hospital.
The Metropolitan Police said in a statement: “A man has been detained at the scene. Enquiries to establish the circumstances and motive are underway.”
Dozens of officers descended on the area, and a wide cordon is in place.
The Prime Minister was being kept “fully informed” of developments, it was reported.
London mayor Sadiq Khan said: “A number of people have been injured. A man has been detained by police. Details are still emerging and I have spoken with, and am in close contact with Assistant Met Commissioner Mark Rowley and our other emergency services.”
A witness told The Independent: “I saw two cars, they crashed. Two men got out of one car and took one man from the other. That’s all I saw.”
Lesley Carter drove past the scene moments after the initial incident.
He told The Independent: “Saw the man pinned down by about six people. One guy was trying to pin him down with his four- or five-year-old daughter in his arms.
“It looked like the car had smashed in to the wall between two other cars and the guy got out to run before being pinned down. Loads of people running away and within minutes the police had turned up.”
Another witness told Sky News: “I was in one of the Chinese restaurants eating. All of a sudden the clientele in the restaurant just hit the floor. Then there were all these people running on Exhibition Road towards the station. A large gathering of police turned up.”
Another witness said: “We were walking near the Science Museum and heard a bang, bang ... At first I thought it was gunfire. Then we walked past the scene of the accident and saw a guy pinned down by other men and lots of people calling the police.
“There didn’t appear to be many people hurt – a few sat by the side of the road but more looking shaken than anything.
“We were then ushered into the Science Museum and the area was quickly cleared of pedestrians.”A BBC reporter said she saw “a car diagonally across the road, looking like it was going into one of the boulders on the side of the road”.
Police asked anyone with information to call 101.
The Natural History Museum tweeted: “There’s been a serious incident outside the Museum. We are working w/ @metpoliceuk and will provide an update when we have more information.”
The A4 Cromwell Road is closed in both directions between Thurloe Place and Queen’s Gate.
South Kensington tube station, initially barricaded by police, has re-opened.
