Prime Minister Theresa May described the incidents as “dreadful events”. The Metropolitan Police tweeted: “At 0025hrs 4/6/17 the incidents at #LondonBridge & #BoroughMarket were declared as terrorist incidents.” London Mayor Sadiq Khan said it was an “horrific terrorist attack”.
He added: “The situation is still unfolding and I would ask all Londoners and visitors to our city to remain calm and vigilant. “We don’t yet know the full details, but this was a deliberate and cowardly attack on innocent Londoners and visitors to our city enjoying their Saturday night. I condemn it in the strongest possible terms.”
Witnesses report hearing gunshots in the Borough Market area while one social media user has posted a photograph appearing to show one of the attackers, with what seemed to be canisters strapped to his body lying on the ground.
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.
The prime minister, who will chair a meeting of the government’s Cobra emergency committee later, said: “Following updates from police and security officials, I can confirm that the terrible incident in London is being treated as a potential act of terrorism.
Mrs May said: “This is a fast-moving investigation. I want to express my huge gratitude to the police and emergency services who are on the scene. “Our thoughts are with those who are caught up in these dreadful events.” Downing Street said Mrs May, who has been campaigning in the general election, was returning to No 10 to receive further briefings from security officials. On Twitter, Labour Party leader Jeremy Corbyn described the incidents as “brutal and shocking”.
Transport for London said London Bridge has been closed in both directions, while neighbouring Southwark Bridge has also been shut. 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.
Meanwhile, Facebook said it has activated its safety check so people in London could post a message to let friends and relatives know they were safe. Police said earlier they were also responding to an incident in the Vauxhall area a few miles away but later confirmed it was not connected to the London Bridge attacks.