LONDON—Britain is under terrorist attack for the third time in less than three months as Scotland Yard warned Londoners to run and hide amid fears that at least one attacker is on the loose in Central London.

Pedestrians were mowed down by a white van that swerved into a crowd on London Bridge. The vehicle continued to Borough market on the southern bank of the River Thames, where witnesses claimed that several men with long-bladed weapons began to attack people in the street and the crowded pubs and restaurants.

As police officers in tactical gear swooped into the popular party district, volleys of automatic gunfire reverberated through the cobbled streets.

Unconfirmed eyewitness reports suggested that the attack was carried out by a gang of terrorists. One survivor showed video footage to a reporter from the Wall Street Journal that reportedly showed the bodies of three attackers who had canisters strapped to their chests.

Officers have not said whether all of the suspected attackers have been apprehended, but they are continuing to operate as though an active manhunt may be underway.

The Mayor of London, Sadiq Khan, said the situation was still unfolding. “The Metropolitan Police are responding to the horrific terrorist attack at London Bridge and Borough Market,” he said. “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. There is no justification whatsoever for such barbaric acts.”

Live television pictures showed one man being detained near the scene shortly before midnight local time (7pm 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 a bombing attack that killed 22 people at an Ariana Grande show just 12 days ago.

Scotland Yard confirmed that they were treating the incidents as acts of terror. Prime Minister Theresa May said: “I can confirm that the terrible incident in London is being treated as a potential act of terrorism.”

Holly Jones, a BBC reporter, said she saw the incident unfold on London Bridge. “A white van driver came speeding—probably about 50mph—veered off the road into the crowds of people who were walking along the pavement,” she told BBC News.

“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.”

As the hunt for at least one attacker continued in Borough market officers raced into bars and restaurants telling patrons to lie on the floor.

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 News of the moment that he realized another terrorist assault on Briton 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.”

Police originally reported that a third incident had occurred further along the south bank in Vauxhall was potentially connected, but it turned out to be an unrelated stabbing.

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.