LONDON—Britain came under terrorist attack for the third time in less than three months on Saturday night as Scotland Yard warned Londoners to run and hide after at least seven people were killed and almost 50 injured by a marauding gang of assailants.

Shortly after 10 p.m. local time, pedestrians were mowed down by a white van that swerved onto the sidewalk on the southern edge of London Bridge. The vehicle continued to Borough Market, a food market in the Southwark neighborhood, three men with long-bladed weapons began attacking people in the street and nearby pubs and restaurants, crowded with Saturday night customers.

As police officers in tactical gear swooped into the popular party district, volleys of automatic gunfire reverberated through the cobbled streets of Central London as they shot down the attackers. The entire incident transpired in just eight minutes.

Police confirmed Sunday morning that seven people were killed in the attack, and that three male suspects had died, although officers continued to operate as though an active manhunt were underway late into the night. "We believe that this incident is under control," Cressida Dick, commissioner of the Metropolitan Police in London, told the press. Officers arrested 12 people in raids on East London on Sunday morning.

A series of four blasts were later heard in the vicinity of London Bridge on Saturday night, although police confirmed that the blasts were controlled demolitions. Police stated that the three deceased attackers “were wearing what looked like explosive vests but these were later established to be hoaxes.”

The mayor of London, Sadiq Khan, was the first to confirm that this was a terrorist attack. “This was a deliberate and cowardly attack on innocent Londoners and visitors to our city enjoying their Saturday night," he said. "I condemn it in the strongest possible terms. There is no justification whatsoever for such barbaric acts.”

No claim of responsibility has been made as yet, and the identities of the attackers have not been revealed, if indeed they are known. European counter-terror sources well-briefed by the British told The Daily Beast that very quickly after the Manchester attack last month the bomber's identity was known, as were the identities of people in his network, who had been watched for some time. If British authorities have such information about the London attackers, they may be reluctant to share it as widely as they did after Manchester, when leaks out of Washington and Paris created potential problems for the investigation.

Live television pictures showed one man being detained near the scene shortly before midnight local time (7 p.m. EST) but it was unclear if he had any involvement in the latest chilling attack to target British civilians enjoying a night out.

The atrocity comes on the eve of a benefit concert which was scheduled to be held in Manchester on Sunday in memory of the victims of the bombing attack that killed 22 people at an Ariana Grande show just 12 days prior.

Prime Minister Theresa May said the terror attack was not believed to be linked to earlier assaults, including a similar attack on Westminster Bridge in March. She pledged a crackdown on radicals within the Islamic community in Britain who have benefited from " far too much tolerance of extremism in our country."

The rampage began on the southern end of London Bridge.

A vehicle mounted the sidewalk, smashing into pedestrians. “A white van driver came speeding—probably about 50 mph—veered off the road into the crowds of people who were walking along the pavement,” said Holly Jones, a BBC reporter, who was on the bridge. “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. I’d say there are about four severely injured people.”

The van then crashed into a lamppost and came to a halt outside a popular pub called the Barrowboy & Banker, which is opposite London Bridge station—one of the city’s busiest transport hubs. The pub stands just above Borough Market, which is arguably London’s finest food market.

With the vehicle now stationary, the attackers leapt out and ran towards the busy bars and restaurants in the streets below, slashing at customers who were gathered outside the Black and Blue steakhouse and the Southwark Tavern.

Security staff at another pub, called the Wheatsheaf, blockaded the entrances as the armed men approached. “The doors were locked by the bar staff. Three men with big hunting knives were outside. They were trying to get in the pub but the doors were locked. They were smashing the windows,” a witness told The Sun.

“I saw at least one man stabbed in the chest about five times.”

Heavily armed officers arrived at the scene just as these assailants were trying to gain entry to the pub. “Armed police turned up, I saw them shoot at least two of the guys in front of me, ” the witness said.

Gabriele Sciotto, an Italian visitor to London, said he saw three men shot by the armed response teams. One of the men, who was lying by a barrier outside the Wheatsheaf appeared to have a series of canisters strapped to his chest.

Sciotto told the BBC that he could not imagine it was a real suicide vest. If it had indeed been a viable explosive device, scores more people could have been killed if the terrorists had succeeded in getting into the packed pub before they were taken out.

At least one British transport police officer was injured as he responded to the attacks.

An eyewitness later told Sky News that he had seen two firearms officers fire automatic weapons into the Brindisa tapas bar on the corner of the market.

Will Heaven, managing editor of the Spectator magazine, told Sky of the moment that he realized another terrorist assault on Britain was unfolding in front of him.

“I was in the back of an Uber cab driving south over London Bridge. Suddenly on the left-hand side on the bridge there was somebody down on the pavement with a small crowd around them, clearly concerned. I thought someone had collapsed.

“We drove a little further over the bridge. There was another person in the road itself. The penny dropped that something quite serious was happening.”

The Department of Homeland Security said it was monitoring the events in London closely, but there was no information to indicate a specific, credible terror threat in the United States.