LONDON -- British police arrested a dozen people Sunday in a widening terrorism investigation after attackers using a van and large knives turned a balmy evening of nightlife into a bloodbath and killed seven people in the heart of London. The Islamic State of Iraq and Syria (ISIS) claimed responsibility.

Although the attackers were also dead, authorities raced to determine whether they had accomplices, and Prime Minister Theresa May warned that the country faced a new threat from copycat attacks.

The country's major political parties temporarily suspended campaigning with only days to go before the general election. May said the vote would take place as scheduled Thursday because "violence can never be allowed to disrupt the democratic process."

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The assault unfolded over a few terrifying minutes late Saturday, starting when a rented van veered off the road and barreled into pedestrians on busy London Bridge. Three men then got out of the vehicle with large knives and attacked people at bars and restaurants in nearby Borough Market until they were shot dead by police.

"They went 'This is for Allah,' and they had a woman on the floor. They were stabbing her," witness Gerard Vowls said.

Florin Morariu, a Romanian chef who works in the Bread Ahead bakery, said he saw people running and some fainting. Then two people approached another person and "began to stick the knife in ... and then I froze and I didn't know what to do."

He said he managed to get near one attacker and "hit him around the head" with a bread basket.

"There was a car with a loudspeaker saying 'go, go' and they (police) threw a grenade. ... and then I ran," he said.

London police said officers killed the attackers within eight minutes of arriving at the scene. Eight officers fired some 50 rounds, said Assistant Commissioner Mark Rowley, the force's head of counterterrorism.

ISIS' statement from its Aamaq news agency claimed the group's "fighters" were responsible, the SITE Intelligence Group said Sunday. IS has urged supporters to weaponize vehicles in attacks against the West.

It was the third attack in Britain this year that ISIS has claimed -- including the similar attack on Westminster Bridge in March and the Manchester concert bombing two weeks ago -- and one of several involving vehicles in Europe, including last year's Bastille Day rampage in the French city of Nice.

The three attackers Saturday were wearing what appeared to be suicide belts, but the belts turned out to be fake. Investigators were working to determine whether others assisted them, Rowley said.

A bystander was also wounded by the gunfire, but the civilian's injuries were not believed to be critical.

Forty-eight people, including two police officers, were treated at hospitals. Twenty-one remained in critical condition Sunday. Among the wounded were German, French, Spanish and Australian citizens, officials said.

Canadian Prime Minister Justin Trudeau said a Canadian woman was among the dead, and a French national was also confirmed dead.

Christine Archibald's family of Canada released a statement saying they grieve the loss of their daughter and sister.

"We grieve the loss of our beautiful, loving daughter and sister. She had room in her heart for everyone and believed strongly that every person was to be valued and respected. She lived this belief working in a shelter for the homeless until she moved to Europe to be with her fiancé," the statement read. "She would have had no understanding of the callous cruelty that caused her death. Please honor her by making your community a better place. Volunteer your time and labor or donate to a homeless shelter. Tell them Chrissy sent you."

Counterterrorism officers raided several addresses in Barking, an east London suburb, and arrested 12 people there Sunday, police said.

The following were arrested, including several women:

[A] 38-year-old woman arrested at address 1 in Barking



[B] 28-year-old man arrested at address 2 in Barking



[C] 52-year-old man arrested at address 2 in Barking



[D] 55-year-old man arrested at address 2 in Barking



[E] 27-year-old man arrested at address 2 in Barking



[F] 55-year-old man arrested at address 2 in Barking, who has since been released without charge



[G] 49-year-old woman arrested at address 2 in Barking



[H] 60-year-old woman arrested at address 2 in Barking



[I] 19-year-old woman arrested at address 2 in Barking



[J] 27-year-old female arrested at address 2 in Barking



[K] 24-year-old female arrested at address 2 in Barking



[L] 53-year-old woman arrested at address 2 in Barking

Neighbors at the site of one raid in Barking said a man who lived there resembled one of the attackers shown in news photographs.

"He's lived here for about three years," Damien Pettit said. "He's one of our neighbors. I've said hello in passing more than 50, 60 occasions. He has two young kids. He was a very nice guy."

Armed officers also conducted a raid in the East Ham area of the city. Video showed police shouting at someone: "Get on the balcony. Stand up and show us your hands!"

The rampage was the third major attack in Britain in the past three months, including a similar vehicle and knife attack on Westminster Bridge in March that left five people dead.

On May 22, a suicide bomber killed 22 people and injured dozens at an Ariana Grande concert in Manchester, in northwest England. Grande and other stars performed Sunday night at a benefit concert for victims under tight security in Manchester.

"I don't feel or smell or hear or see any fear in this building. All we feel here tonight is love, resilience, positivity," said Pharrel Williams, who performed alongside Miley Cyrus.

May said the London and Manchester attacks were not directly connected, "but we believe we are experiencing a new trend in the threat we face" as "terrorism breeds terrorism" and attackers copy one another. She said five credible plots have been disrupted since March.

"It is time to say, enough is enough," she said.

Britain's official terrorism threat level was raised from "severe" to "critical" after the Manchester attack, meaning an attack may be imminent. Several days later it was lowered again to "severe," meaning an attack is highly likely.

Home Secretary Amber Rudd said Sunday that the level would remain at severe because police believe there are no perpetrators still on the loose.

London Bridge and a large area on the south bank of the River Thames remained cordoned off Sunday, and police told people to avoid the area.

Hours earlier, the area packed with bars and restaurants around the foodie magnet of Borough Market had been a scene of panic, as people barricaded themselves in pubs and restaurants or fled through the streets.

Medics treated the wounded near the market as shocked people cried and shouted around them. Police officers yelled at people to run from the area, and blasts were heard as officers performed a series of controlled explosions.

Renan Marquese, a sous-chef at a tapas restaurant, said he was working when he heard chaotic sounds outside.

"When I open the door I see three dead people on the floor," he said. "People running everywhere, police shouting to run away."

He said that he helped a man and his partner, even taking the woman into his arms because she was too upset to walk properly. He said it took him 20 minutes to carry her across the bridge, stumbling all the way.

"It was really scary," he said.

Amid the violence and fear were stories of compassion and heroism. The British Transport Police said one of their officers, among the first to arrive, took the attackers on armed only with his baton and was seriously wounded. He was later described as being in stable condition with injuries that were not life-threatening.

Witnesses described how passers-by threw chairs and beer glasses at the attackers in an attempt to stop them.

Richard Angell, who was in a restaurant, said he looked out and saw "a guy who is throwing a table at somebody, and it's very unclear about what is happening. And it turns out to be a heroic guy who saw what was happening and just bombarded these terrible cowardly people with stuff."

Vowls also saw people striking back at the attackers and said he joined in.

"I went 'Oi, terrorists, cowards, Oi!'" he told The Associated Press. Then he picked up a chair.

"I chucked it, but I think I missed one of them, and then I picked up a stool, and I threw it at him. And he looked at me. He started running towards me, and then he decided not to.

"Then I was screaming at them, picking up bottles from a beer barrel. I was just throwing it at them, trying to get them to chase me so I could get them out into the main road where the police could see them and obviously take them down."

The U.S. Embassy in London remains in close contact with London authorities and continues to gather information, which includes any updates about U.S. citizens who may have been injured. At this time, officials are not aware of any U.S citizens killed. The U.S. Embassy in London said it stands ready to provide assistance to any U.S. citizens affected by this tragedy.

The United Kingdom will observe a moment of silence on Tuesday at 11 a.m. local time to reflect on the lives lost in the terror attack.