LONDON -- Prime Minister Theresa May called Sunday for tougher measures to contain Islamic extremism in Britain in the wake of an attack that killed seven people in a busy area of London.

The assault began Saturday night when a van veered off the road and barreled into pedestrians on busy London Bridge. Three men fled the van with large knives and attacked people at bars and restaurants in nearby Borough Market, police and witnesses said. The attack unfolded quickly, and police said officers had shot and killed the three attackers within eight minutes.

Police said counterterrorism officers raided several addresses in Barking, an east London suburb, and arrested 12 people there Sunday morning in connection with the attack. Armed officers also conducted a raid in the East Ham area of the city.

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Emergency officials said 48 people, including two police officers, were treated at London hospitals and a number of others suffered less serious injuries. Twenty-one remained in critical condition Sunday. Among the wounded were German, French and Spanish citizens, officials said.

The death toll of seven did not include the three attackers. Canadian Prime Minister Justin Trudeau said a Canadian was among the dead.

Britain was already on edge after a suicide bombing two weeks ago at an Ariana Grande concert in Manchester, in northwest England, that killed 22 people and injured dozens of others. Grande and other stars are scheduled to perform a benefit concert for victims Sunday night.

The Islamic State group claimed responsibility for the Manchester bombing, but there has not yet been a claim of responsibility for the London attack, which the prime minister linked to Islamic extremism.

May, facing an election Thursday, said the recent attacks, including an earlier one outside Parliament in March, are not directly connected, but that "terrorism breeds terrorism" and attackers copy one another. She also said five credible plots have been disrupted since March.

"They are bound together by the single evil ideology of Islamist extremism that preaches hatred, sows division, and promotes sectarianism," she said. "It is an ideology that claims our W It was the third attack to hit Britain in as many months.

In March, a British convert to Islam ran down people with a vehicle on Westminster Bridge, killing four before fatally stabbing a policeman on Parliament's grounds.

Then came the May 22 Manchester concert bombing. After that attack, Britain's official terrorism threat level was raised from "severe" to "critical," meaning an attack may be imminent. Several days later it was lowered again to "severe," meaning an attack is highly likely.

May called on international communications companies to do more to block cyberspace to extremist groups who use it for recruitment and for encrypted information about plots. She called for international agreements to regulate cyberspace and said Britain needs to become more robust at preventing the internet from being used to the advantage of extremist groups.

The British government has long sought more help from internet companies like Facebook and Google in the battle against extremism.

May spoke defiantly about protecting Britain's democracy -- and vowed the election would go ahead as planned -- after the violence turned a balmy summer night in an area packed with revelers into a scene of bloodshed and chaos, with officers running through crowded streets screaming for people to flee.

Britain's Prime Minister Theresa May speaks outside 10 Downing Street after an attack on London Bridge and Borough Market left 7 people dead and dozens injured in London, Britain, June 4, 2017. REUTERS/Hannah Mckay

Boats on the River Thames helped evacuate the area, which is popular with tourists. It remained closed off Sunday as police urged residents and tourists to stay away.

May said the men attacked "innocent and unarmed civilians" in crowded Borough Market with blades and knives. She said they were wearing what appeared to be explosive vests, but police determined those were only meant to sow panic and fear.

Major parties suspended national campaigning Sunday out of respect for the victims, although the rightwing UK Independence Party said it would keep campaigning to show the extremists they couldn't sidetrack democracy.

May's Conservative Party had been expected to win by a wide margin but recent polls have showed the race tightening considerably. It is unclear how the unprecedented violence in the run-up to the election will impact voter sentiment.

Mayor Sadiq Khan said Londoners should remain vigilant but added: "I'm reassured we are one of the safest global cities in the world."

Khan said some of the injured were in critical condition. French and Spanish citizens were among the wounded.

Assistant Metropolitan Police Commissioner Mark Rowley said that while police believe all the attackers were killed, the investigation is expanding.

"We believe three people were involved, but we still have got some more inquiries to do to be 100 percent confident in that," he said.

U.S. President Donald Trump tweeted about the attack early Sunday, saying, "We must stop being politically correct and get down to the business of security for our people. If we don't get smart it will only get worse."

We must stop being politically correct and get down to the business of security for our people. If we don't get smart it will only get worse — Donald J. Trump (@realDonaldTrump) June 4, 2017

"At least 7 dead and 48 wounded in terror attack and Mayor of London says there is 'no reason to be alarmed!'" Mr. Trump tweeted.

"Do you notice we are not having a gun debate right now? That's because they used knives and a truck!" he posted.

At least 7 dead and 48 wounded in terror attack and Mayor of London says there is "no reason to be alarmed!" — Donald J. Trump (@realDonaldTrump) June 4, 2017

Do you notice we are not having a gun debate right now? That's because they used knives and a truck! — Donald J. Trump (@realDonaldTrump) June 4, 2017

Hours earlier, as authorities responded to reports of a string of attacks, Mr. Trump argued in favor of his embattled travel ban.

"We need to be smart, vigilant and tough," he tweeted. "We need the courts to give us back our rights. We need the Travel Ban as an extra level of safety!"

He also tweeted a message of support for Londoners: "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!"

As dawn broke over London Sunday, a large area on the south bank of the River Thames remained cordoned off. Police told people to avoid the area, leaving tourists and revelers struggling to get home. London Bridge and the adjacent train station, as well as nearby Borough subway station, were still closed.

Only hours before, bursts of gunfire echoed through the streets -- likely from armed police -- and at least three blasts rang out as officers performed controlled explosions after sweeping the area for possible bombs.

One witness, identified only as Ben, told the BBC that he and his wife saw a stabbing at Borough Market. "I saw a man in red with quite a large blade -- I don't know the measurement, I guess maybe 10 inches," he said. "He was stabbing a man. He stabbed him about three times fairly calmly."

Before fleeing the scene, Ben said he saw someone throw a table and a glass bottle at the suspect.

Another witness, identified as Gerard, told the BBC that he chased the suspects and threw bottles, glasses and other objects in an effort to stop them.

"They were running in all the pubs and all the bars and they were stabbing everyone," he said. "I was throwing bottles at them, pint glasses, stools, chairs."

Bartender Alex Martinez said he hid in a garbage bin for a half hour when a man stormed the nearly full restaurant where he worked. "I saw that man with a knife in his hand and after that a man started screaming so I knew something wrong was happening," he said.

Medics treated the injured near the market as shocked people cried and shouted around them. Police officers yelled at people to run from the area.

Simon Thompson told Sky News that he was just outside Borough Market when he saw crowds fleeing.

"We ran for like 100 meters and then saw loads of police cars turned up and there was kind of a period of quite intense gunfire," he said. "I hid in a restaurant basement for about an hour... Police told us to get out and then there was more gunfire."

Police received the first call of reports of a vehicle hitting pedestrians on London Bridge at 10:08 p.m.

Soon after, reports started coming in of stabbings at Borough Market, a nearby area full of bars and restaurants surrounding a popular food market.

Photographer Gabriele Sciotto said he was on his way home from a pub when he saw a man running toward him telling him to turn around and run because there was an attack. As a documentary filmmaker, though, his instincts were to keep going.

The 25-year-old said that at first, "it didn't look too dangerous."

The men went into Borough Market, but then turned around and ran toward the Wheatsheaf Pub after being confronted by a police officer, he said, and suddenly a lot of police came from the other direction.

The attackers "had no clue what they were doing. They were scared. The police were scared," he said.

After one of the men was shot, Sciotto captured a photo of the suspect on the ground wearing what appeared to be canisters.

Police initially said officers were also responding to a third incident, in the Vauxhall neighborhood, but later said that turned out to be an unrelated stabbing.

Police tweeted a warning telling people in the area to run to safety, hide and then call police if it was safe to do so. They asked Londoners to "remain calm but be alert and vigilant."