LONDON (CBSNewYork/CBS News/AP) -- ISIS has claimed responsibility for Saturday night's terror attack in London.
In a statement more than 24 hours after the attack, the group's Aamaq news agency quoted a "security source," saying, "A Unit of Islamic State Fighters carried out London attacks yesterday."
The death toll had risen to seven Sunday morning – in addition to three suspects who were shot and killed by police.
At least four dozen were wounded, and police have arrested 12 people, one of whom has since been released.
As CBS News' Teri Okita reported, the events unfolded around 10 p.m. GMT when a white van mowed down pedestrians along the London Bridge, police said. Police said three suspects exited the vehicle at the nearby Borough Market and attempted to stab people.
Witness Gerard Vowls said he was in a pub trying to throw tables and beer glasses at the suspects.
"They went, 'this is for Allah,' and just started stabbing her multiple times," he said. "Some guy comes around the corner and went, 'run, run, run they got blades, knives, they are going to stab you. They are terrorists. Run.' And then he turned around, and then they just stabbed him multiple times, as well."
Dozens of stunned people left the area with their hands on their heads as heavily armed police fanned across the crowded capital.
"I just saw loads of people run away from the market and there was people lying on the ground, and there was a taxi driver who rolled his window down and was shouting at people to run," said witness Simon Thompson.
Emergency officials said 48 people were treated at London hospitals and a number of others suffered less serious injuries.
As of Sunday afternoon local time, 36 people remained hospitalized and 21 remained in critical condition, police said.
The three suspects, who were wearing hoax suicide vests, were shot and killed by police within eight minutes.
A photo showed one suspect lying on the ground with the canisters on his belt.
"The suspects were wearing what looked like explosive vests, but these were later established to be hoaxes," said Metropolitan Police Assistant Commissioner Mark Rowley.
But Rowley said later Sunday that eight officers fired an estimated 50 rounds, believing they were confronting attackers with suicide belts.
"The situation these officers were confronted with was critical -- a matter of life and death," he said.
The British Transport Police chief said an officer who was wounded in the London Bridge attack was armed only with his baton when he confronted the three knife-wielding assailants. The officer, who was stabbed in the face, head and a leg, was one of the first responders.
British Transport Police Chief Constable Paul Crowther visited the officer at a hospital and said that he's in stable condition. Crowther said that "it became clear that he showed enormous courage in the face of danger." Crowther added that "for an officer who only joined us less than two years ago, the bravery he showed was outstanding and makes me extremely proud."
On Sunday morning, police arrested 12 people in East London. Police said in a statement that the arrests were made Sunday morning in Barking by counterterrorism officers.
"Work is ongoing to understand more about their connections and about whether they were assisted or supported by anyone else," Rowley said.
Police have not released the names of the killed terrorists, but did say one of them rented the van that was used in the attack, CBS2's Brian Conybeare reported.
The area around London Bridge and Borough Market was to remain cordoned off following the attack, Rowley said. The public in London can also expect to see more police officers, armed and unarmed, around the city.
Flowers were left on the sidewalk as shaken witnesses returned to the scene on Sunday.
"You hear Westminster, Manchester and you always think it could be anytime, it could be us," witness Caine Pieri said. "And, touch on wood, it won't happen again."
Meanwhile, Prime Minister Theresa May called Sunday for tougher measures to contain Islamic extremism in Britain.
"It is time to say enough is enough," she said.
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 performed a benefit concert for victims Sunday night.
ISIS also claimed responsibility for the Manchester bombing.
"The recent attacks are not connected, but we believe we are experiencing a new trend in the threat that we face, as terrorism breeds terrorism," May said. "They are bound together by the single, evil ideology of Islamist extremism."
May noted that the London attack was the third 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.
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.
May also paid a private visit to some of the victims of the vehicle and knife attacks in central London. May visited King's College Hospital, which was caring for 14 of the people hospitalized.
The National Health Service says 21 people remain in critical condition.
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.
President Donald Trump tweeted about the attack early Sunday, railing against "political correctness" and noting that the attack involved weapons other than guns. He also appeared to criticize Mayor Khan's remarks.
Later Sunday, the president spoke about the attack, making a simple vow.
"This bloodshed must end. This bloodshed will end," Trump said.
The NYPD said late Saturday that it was closely monitoring the situation in London, but added that "there are no specific, credible threats to NYC."
"We are monitoring the events in London and have been in touch with the British authorities. We will make adjustments to our deployments in New York City, as needed. As always, we ask any member of the public who observes suspicious behavior to contact the police immediately," the NYPD said in a statement.
(© Copyright 2017 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)
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