More than one person has died after an incident involving a vehicle and stabbings in the London Bridge area of the capital, police have said.
Armed police responded to reports of a van hitting pedestrians on the bridge in central London.
The Met Police said officers were also at Borough Market nearby where they were responding to reports of stabbings in the restaurant and bar venue.
The force said it was also attending a reported incident in Vauxhall.
Downing Street said Prime Minister Theresa May, who has been campaigning in the general election, was returning to No 10 to receive further briefings from security officials.
It said the prime minister will chair a meeting of the government’s Cobra emergency committee later.
Meanwhile, Facebook said it has activated its safety check so people in London could let friends and relatives know they were safe.
Transport for London said London Bridge has been closed in both directions due to a “major police incident”.
Bus routes were being diverted and the neighbouring Southwark Bridge has also been shut, it added.
TfL said there were further closures in Borough High Street, where armed police and counter-terrorism specialist forces were later seen. On the north side of the river, Lower Thames Street was closed.
BBC home affairs correspondent Tom Symonds said a security guard who oversees a number of pubs in the London Bridge area said he saw four people stabbed by three attackers.
BBC reporter Holly Jones, who was on the bridge, said a van driven by a man was “probably travelling at about 50 miles an hour” before it hit a number of people.
“He swerved right round me and then hit about five or six people. He hit about two people in front of me and then three behind,” Ms Jones told the BBC News Channel.
Five or six people were being treated for injuries after the vehicle mounted the pavement and hit them, she said.
“I’d say there are about four severely injured people. They all have paramedics assisting them at the moment.”
She said the van, which was travelling from the direction of central London, headed towards the south side of the river.
Ms Jones later reported seeing a man being arrested by police. She said he was handcuffed and had his shirt off.
She said a French woman was among the injured. She had told said she did not know where two people who had been with her were.
Speaking to the Press Association, Will Orton described being in a pub in the area and seeing people coming running inside.
“We didn’t really know what was going on,” he said.
“We thought maybe there was a fight or something outside. And then there were almost hundreds of people coming inside.
“The bouncers did a really good job, they shut the doors and locked everyone in. There was panic – it seemed like it was literally outside the door. People were coming inside and saying they had witnessed people being stabbed.”