Suspected militants drove a van into pedestrians at high speed on London Bridge on Saturday night before stabbing revellers on nearby streets, killing at least six people and wounding close to 50.
Armed police rushed to the scene and shot dead the three male attackers in the Borough Market area near the bridge, as authorities urged Londoners on Twitter to “run, hide, tell” if they were caught up in the violence.
“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,” May said in a statement as events unfolded.
The killings also occurred less than two weeks after a suicide bomber killed 22 people at a pop concert by U.S. singer Ariana Grande in Manchester in northern England.
There was no immediate claim of responsibility.
“Sadly, six people have died in addition to the three attackers shot by police,” Mark Rowley, Britain’s top anti-terrorism officer, said. The three attackers had been wearing what looked like explosive vests that were later found to have been hoaxes.
The BBC showed a photograph of two possible attackers shot by police, one of whom had canisters strapped to his body. Hours after the attack the area remained sealed off and patrolled by armed police and counter-terrorism officers.
The London Ambulance Service said 48 people had been taken to five hospitals across the city and a number of others had been treated at the scene for minor injuries.
Streets around London Bridge and Borough Market, an area that is a major transport hub and is packed with bars and restaurants, was busy with people on a night out.
London Mayor Sadiq Khan spoke of a “cowardly attack on innocent Londoners and visitors to our city enjoying their Saturday night”.