Several people have been injured after a car hit pedestrians near the Natural History Museum in London, police say.
The Metropolitan Police said one person had been detained following the incident at 14:20 BST in Exhibition Road in South Kensington.
Video footage that emerged on Twitter showed a man being restrained on the ground.
London Ambulance said it treated 11 people, mostly for head and leg injuries. Nine were taken to hospital.
Police have not indicated the nature of the incident, saying inquiries are under way to establish the circumstances.
A Downing Street spokesman said the prime minister is “being kept up to date on events”.
Labour leader Jeremy Corbyn tweeted: “Very concerning reports from outside the Natural History Museum. My thoughts are with those injured and our thanks to the emergency services.”
London Mayor Sadiq Khan tweeted that details were “still emerging” and he was in close contact with Assistant Metropolitan Commissioner Mark Rowley – the head of UK counter-terrorism policing.
“Police have asked people to avoid the area while their inquiries continue,” he added.
The current terror threat level in the UK is “severe” – the second highest – meaning an attack is highly likely.
BBC reporter Chloe Hayward, who was leaving the Natural History Museum as the incident happened, said she saw a car “diagonally across the road”, looking like it had hit a bollard.
“I could see a crowd of people around what was clearly one or two people on the pavement,” she said.
She then saw armed police arrive within minutes.
“We have had lots of police coming onto the scene, helicopters above, and I can see an ambulance which is definitely having someone put in, but it isn’t clear how bad that injury is.”
We have been to the south end of Exhibition Road nearest the Tube and the area, normally a busy destination for Saturday afternoon dining by locals and tourists, is deserted.
Eyewitnesses told us that police came rushing into each bar and restaurant and told people to get out.
We can see coats on chairs – some knocked over – half-eaten meals and half-drunk glasses of wine.
Police helped one restaurant owner to recover staff belongings, like house keys, because it’s unclear when the area will reopen.
Exhibition Road is an area popular with tourists as it is home to the Natural History Museum, the Science Museum and the Victoria and Albert Museum.
An eyewitness who was walking to the Science Museum said: “When waiting for the light, we heard what I thought was gunshots and saw a car drive over the pavement. We just ran. My friend dived on the floor and cut her hands.”
The woman, who asked to remain anonymous, said: “When it calmed down we walked back to where we’d been and saw a gentleman on the floor being restrained by police.”
Connor Honeyman, from Essex, who was in the queue for the museum, said: “We heard a horrible thudding noise and a car engine. Everyone started running and screaming inside.
“We ran in, everyone was following us, and then all the security guards ran out and they closed the main entrance. There was much confusion before the police got there.”
Joseph Degraff, 23, who is on holiday from Washington DC, was in a cafe on Exhibition Road and said: “We saw a large stampede of people running and thought, ‘what’s going on?’.
“Someone who works in the cafe said ‘get on the ground’. We hit the floor. We were there for a few seconds, then they told everyone to get in the basement.
“At that point I thought it was a shooting or a bomb or something. We just didn’t know.
“The police told everyone to get out of the cafe and we got out and ran.”
An spokeswoman from the Natural History Museum said: “The Museum’s visitors and staff are always our first priority.
“We are working with emergency services to assess the incident and we are doing all we can.”