Six people have been killed in an attack in central London in which three assailants were shot dead by police, Scotland Yard has said.
A white van hit several pedestrians on London Bridge just after 22:00 BST.
Three men then left the vehicle and stabbed a number of people in Borough Market, an area known for its restaurants and bars.
Armed officers confronted and shot three male suspects within eight minutes.
Met Police Assistant Commissioner Mark Rowley said: "The suspects were wearing what looked like explosive vests but these were later established to be hoaxes."
He said it was currently believed there were only three attackers involved.
London Ambulance Service said at least 30 people have been taken to hospital.
The British Transport Police said one of its officers was seriously injured after being stabbed as he responded to the incident, but his condition was not said to be life-threatening.
Prime Minister Theresa May described the incidents as "dreadful events" and will chair a meeting of the government's Cobra emergency committee later on Sunday.
She added: "I can confirm that the terrible incident in London is being treated as a potential act of terrorism."
London Mayor Sadiq Khan called it "a deliberate and cowardly attack on innocent Londoners".
Eyewitnesses told BBC home affairs correspondent Tom Symonds they saw people getting out of the van after the attack on the bridge and running towards Borough Market.
Gunshots were later heard in the area while one social media user posted a photograph appearing to show one of the attackers lying on the ground outside a pub in the food market, with what seemed to be canisters strapped to his body.
The attack comes almost two weeks after 22 people died in a suicide bombing at a concert in Manchester. In March, five people died in London when a car was driven at pedestrians on Westminster Bridge and a police officer was stabbed outside Parliament.
The BBC's 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 at the time of the attack, 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.
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