A careening van plowed into pedestrians Saturday night on one of London's iconic bridges before attackers leaped from the vehicle and began stabbing patrons in a nearby nightlife area, witnesses said, in what police described as the country's third major terrorist attack in a three-month span.
Scotland Yard said six people died in the attacks and that three suspects were shot and killed.
Chaos erupted when a white van rammed into pedestrians on London Bridge, injuring an undetermined number of people and causing hundreds of others to flee in panic.
Shortly after midnight local time, London's Metropolitan Police Service tweeted that the violent episodes on the bridge and nearby Borough Market, crammed with pubs, clubs and restaurants, were being treated as terrorist incidents.
Earlier, Prime Minister Theresa May said the violence was being treated as a "potential act of terrorism," British media reported. Peter Rhodes, assistant director of operations at the London Ambulance Service, said in a statement that at least 30 patients had been taken to five hospitals.
"We have also treated a number of people at the scene for less serious injuries," he said.
The London attack comes at a politically sensitive moment and when security fears are running high. On Thursday, British voters are set to head to the polls for a general election. In May — for the first time in 10 years— Britain raised its terrorist threat level from "severe" to "critical" in the wake of a deadly bombing in Manchester.
As authorities searched for suspects, police announced on Twitter that officers were also responding to reports of stabbings in Borough Market — not far from the bridge. Police reported shots had been fired.
Patrons who had been enjoying an early-summer night on the town described a scene of horror. Alex Shellum, in a pub just below the bridge, told the BBC he saw a young woman staggering into the pub, bleeding from the neck.
"Her throat had been cut," he said. The pub was swiftly closed, and fleeing patrons saw medics treating another victim.
Those traveling on the bridge above caught a swift and vivid glimpse of unfolding carnage. Connor Morris, 20, was on the upper level of a double-decker public bus when it stopped suddenly. He thought the cause must have been a traffic accident.
"Out of the corner of my eye, I saw something moving," Morris, a student at the Royal Central School of Speech and Drama, said in a telephone interview. "I saw a white van and four bodies in front of me and people were trying to resuscitate them."
He left the bus with his friend, and a blood-covered man urged the pair to flee.
"I feel numbness right now," he said.
Holly Jones, a BBC reporter who happened to be at the bridge when the incident occurred about 10:30 p.m., described what happened for BBC News: "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."
"He swerved right around me and then hit about five or six people," she added. "He hit about two people in front of me and then three behind."
City transportation officials announced traffic on the bridge was shut down in both directions.
A tweet from the police urged people in the area to "Run, Hide, Tell."
The attack, coming less than two weeks after the suicide bombing of a pop concert in the northern city of Manchester, drew expressions of concern and condolences from around the world.
President Trump spoke with May, offering his condolences and offering help in the investigation.
He also conveyed sympathy and solidarity via Twitter: "Whatever the United States can do to help out in London and the U. K., we will be there - WE ARE WITH YOU. GOD BLESS!"
But Trump also set off a storm of criticism on social media when he suggested that the attack bolstered the argument for his proposed travel ban, which is under legal challenge.
"We need to be smart, vigilant and tough. We need the courts to give us back our rights," he tweeted. "We need the Travel Ban as an extra level of safety!"
The U.S. State Department denounced "cowardly attacks targeting innocent civilians in London this evening."
"Our hearts are with the families and loved ones of the victims," said State Department spokeswoman Heather Nauert. "We wish a full and quick recovery to those injured in the attacks. All Americans stand in solidarity with the people of the United Kingdom."
For those enjoying a Saturday night out in London, an evening's revelry quickly turned into a frightening ordeal. Paige Jegermanis, a 22-year-old American living in London, said that soon after the bridge episode, a bartender informed the crowd in a riverside pub: "We're all on lockdown."
Half an hour later, they were asked to leave. The scene outside was "absolutely insane," she said. "Cop cars were going by, sirens on, people were running, I was crying."
Calum Curry told CNN that he and friends were walking near the bridge Saturday night when he heard a loud noise.
"We saw a car and a few bodies," Curry said. "Police flooded the area really quickly. … We heard a bit of gunshots."
London Mayor Sadiq Khan issued a statement calling the incident "a deliberate and cowardly attack on innocent Londoners and visitors to our city."
"I condemn it in the strongest possible terms. There is no justification whatsoever for such barbaric acts," he said.
Last month, 22 people were killed and more than 59 others were injured in an explosion at an Ariana Grande concert in Manchester that police said was caused by a bomber carrying an improvised explosive device. The explosion happened near an entrance to the 21,000-seat arena just minutes after Grande's concert ended.
Saturday's attack was a chilling echo of a strike in March, when an assailant drove a car into pedestrians on Westminster Bridge in London, killing two, and then stabbed a police officer to death before being shot to death himself.
This spring the U.S. Transportation Security Administration issued a report titled "Vehicle ramming attacks: Threat landscape, indicators and countermeasures," which notes that in the last three years, at least 173 people have been killed and more than 700 wounded in 17 ramming attacks around the world.
In December, a Tunisian man with ties to Islamic State deliberately drove a 27-ton truck into a Christmas market in Berlin, killing 12 people and wounding 56 others. And last July, as thousands crammed into the streets of Nice, France, for a Bastille Day celebration, another assailant influenced by Islamic State drove a 19-ton cargo truck into a crowd, leaving 86 dead and 434 injured.
Times staff writers Christina Bellantoni and David Montero contributed to this report.
7:45 p.m.: This article has been updated with eyewitness accounts and comment from the U.S. State Department.
5:45 p.m.: This article has been updated with police treating the attack as terrorist incident, details from a witness, a statement from London's mayor and additional background.
4:45 p.m.: This article has been updated with statements from Theresa May and President Trump, reports of fatalities and other details.
3:55 p.m.: This article has been updated with additional details and quotes.
3:30 p.m.: This article has been updated with details from BBC, more background.
3:05 p.m.: This article has been updated with background on vehicle ramming attacks.