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(AFP) - Police carried out fresh raids and arrested "a number of people" on Monday after the Islamic State group claimed an attack by three men who mowed down and stabbed revellers in London, killing seven people before being shot dead by officers.

Saturday night's rampage at a popular nightlife hub around London Bridge by men wearing fake suicide vests was the third deadly terror attack in Britain in less than three months and came only days before a general election.

"A number of people have been detained," police said in a statement after two early morning raids in east London, as commuters returned to the scene of the attacks after some security cordons were removed.

"A very high priority for us is to try to understand whether they were working with anybody else," London police chief Cressida Dick told BBC television.

Dick said police had seized "a huge amount of forensic material" after going through the van used in the attack "very very carefully".

"We will change and adapt to what appears to be a new reality for us," she said.

British Prime Minister Theresa May on Sunday blamed "evil" Islamist ideology and vowed to crackdown on extremist content online worldwide, warning that attackers were "copying one another".

National campaigning for Thursday's general election resumed on Monday after a one-day suspension out of respect for the victims, who included 48 people treated in hospital for injuries.

Of those, 21 are still in a critical condition.

Police said on Sunday they were holding 11 people, all arrested in raids on two addresses in Barking in suburban east London.

The Islamic State group claimed responsibility.

"A detachment of fighters from Islamic State carried out London attacks," said the Amaq news agency, which is affiliated with the jihadists.

One Canadian national and one Frenchman were among the fatalities and seven French citizens were among the injured.

No details have been released about the perpetrators, who were shot dead within eight minutes of the first call to the police.

Eight officers fired an "unprecedented" 50 rounds at the three attackers, according to Mark Rowley, head of national counter-terrorism policing, who said that a bystander had also suffered a gunshot wound.

- 'Copying one another' -

The prime minister said the attack was driven by the same "evil ideology of Islamist extremism" behind the May 22 Manchester suicide bombing that left 22 people dead, and the Westminster attack in March, which killed five.

The assailants ran people over on London Bridge before lunging seemingly at random at the crowds gathered around Borough Market, which is full of restaurants and bars.

Gerard Vowls, 47, said he saw a woman repeatedly stabbed, and threw chairs, glasses and bottles at the attackers in a bid to stop them.

"They kept coming to try to stab me... they were stabbing everyone. Evil, evil people," he told The Guardian newspaper.

Another witness called Eric told the BBC he had seen three men get out of the van who took out knives.

"It was a rampage," he said, adding that he heard a shout of: "This is for Allah".

An Australian was among those hospitalised, while a Spaniard escaped with light injuries.

French Foreign Minister Jean-Yves Le Drian was due to visit Britain on Monday to speak to the injured French nationals, after President Emmanuel Macron said France was "more than ever at Britain's side".

Among those stabbed was a British Transport Police officer, who was one of the first responders on the scene and received injuries to his face.

A vigil for the victims will take place at nearby Tower Bridge on Monday evening.

- 'Praying for London' -

Britain was already on high alert following the recent attack on a concert by US pop star Ariana Grande in Manchester, northwest England, in which seven children were among the 22 dead.

Grande, who headlined a benefit concert in Manchester on Sunday, alongside stars including Pharrell Williams and Justin Bieber, tweeted that she was "Praying for London".

"A lot of people suffered and I had some second thoughts coming, especially with what happened last night in London," ticket holder Abdullah Mala, 34, told AFP. "But we've got to move on".

The national threat level was raised to maximum after the Manchester attack and troops were deployed at key public sites, but reduced back down to its second-highest level last weekend.

Another meeting of Britain's Cobra emergency cabinet was to be held on Monday.

May, who served as interior minister for six years before taking office in July after the Brexit vote, said Britain's response to the terror threat must change.

"We cannot and must not pretend that things can continue as they are," she said, adding there was "far too much tolerance of extremism in our country."

The governing Conservatives and the main opposition Labour party suspended national campaign events for the day, but May insisted the election would go ahead as planned on Thursday.

Saturday's rampage is the latest in a string of attacks to hit Europe, including in Paris, Berlin and Saint Petersburg, and the French, German and Russian leaders sent messages of support.

US President Donald Trump offered his help, tweeting "WE ARE WITH YOU. GOD BLESS!" -- and highlighting his thwarted ban on travellers from six mainly Muslim countries.

© Agence France-Presse