British police said incidents at London Bridge and the nearby Borough Market area on Saturday were terrorist incidents.
But they said a separate stabbing in the Vauxhall area was not terrorism related.
“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,” Prime Minister Theresa May said.
Attackers had driven a van at high speed into pedestrians on London Bridge before stabbing people in the nearby Borough Market area of bars and restaurants.
Armed police rushed to the scene where shots were heard and authorities urged Londoners on Twitter to “run, hide, tell” if they were caught in the attack. The BBC cited police as saying there had been more than one fatality.
Britain’s Sun newspaper said seven people were feared killed and that two attackers were shot dead by police near London Bridge; but there was no immediate confirmation of this. Some media reports said police were seeking another attacker.
The attacks come days ahead of a June 8 election and 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.
The BBC showed a photograph of two possible London attackers shot by police, one of whom had canisters strapped to his body. Hours after the attack the area remained sealed off.
The London ambulance service said at least 20 people had been taken to six hospitals across the city. Three major London hospitals said that they were on lockdown to keep patients and staff safe.
A Reuters reporter said some time after the attack began that he had heard loud bangs near the Borough Market area.
Streets around London Bridge and Borough Market, fashionable districts packed with bars and restaurants, would have been busy with people on a Saturday night out. BBC showed dozens of people, evidently caught up in the attack, being escorted through a police cordon with their hands on their heads.
Witnesses described a white van careering into pedestrians near London Bridge and knocking over several people.
“It looked like he was aiming for groups of people. I froze because I didn’t know what to do,” Mark Roberts, 53, a management consultant, told Reuters. He saw at least six people on the ground after the van veered on and off the pavement.
“It was horrendous,” he said.
A taxi driver told the BBC that three men got out of the van with long knives and “went randomly along Borough High Street stabbing people.”
Witnesses said people ran into a bar to seek shelter.
“People started running and screaming, and the van crashes into the railing behind. We went toward Borough Market and everyone went inside (the bar). Everyone in the bar started pushing people from the exits,” one witness who gave his name as Brian, 32, told Reuters.
Another witness, who declined to be named, his white top covered in blood, described a scene of panic in the bar.
“They hit the emergency alarm. There was a line of people going down to the emergency exit. And then people started screaming coming back up,” the 31-year-old said.
“Around the corner there was a guy with a stab wound on his neck … There was a doctor in the pub and she helped him. They put pressure on the stab wound.”
BBC radio said witnesses saw people throwing tables and chairs at the perpetrators of the attack to protect themselves.
ISLAMIC STATE CALL
Islamic State earlier on Saturday sent out a call on instant messaging service Telegram urging its followers to launch attacks with trucks, knives and guns against “Crusaders” during the Muslim holy month of Ramadan.
Similar attacks, in Berlin, Nice, Brussels and Paris, have been carried out by militants over the past couple of years.