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London (AFP) - British Prime Minister Theresa May blamed "evil" Islamist ideology Sunday for an attack by knife-wielding men who mowed down and stabbed revellers in London, killing seven, as police said they had arrested 12 suspects.

Saturday night's rampage at the popular nightlife hub around London Bridge, by three men arriving in a van and wearing fake suicide vests, was the third deadly terror attack in Britain in three months and came only days before snap elections.

National campaigning for Thursday's general election was suspended for the day out of respect for the victims, who included 48 people treated in hospital, some of them in life-threatening conditions.

No details have been released about the suspects, who were shot dead within minutes by police, and detectives are still investigating whether they acted alone.

The 12 arrests were made in the ethnically diverse east London suburb of Barking, with Sky News reporting that a property raided by police belonged to one of the killers.

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

"The recent attacks are not connected but we believe we are experiencing a new trend in the threat we face," she said after chairing a meeting of the government's emergency Cobra committee.

She warned that perpetrators are inspired to attack "by copying one another".

The assailants wore fake suicide vests in a bid to increase the sense of panic as they lunged seemingly at random at the crowds gathered around London Bridge and 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.

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Holly Jones, a BBC reporter, saw a white van speeding into crowds of people walking along the pavement on London Bridge, saying it hit about five or six people.

Another witness called Eric told the BBC he had seen three men get out and thought they were going to help.

Instead they "started kicking them, punching them and took out knives. It was a rampage really," he said, adding that he heard a shout of: "This is for Allah".

An Australian and four French nationals were among those hospitalised, their governments said, while a Spaniard was slightly wounded.

- 'Praying for London' -

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

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

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

May, who served as interior minister for six years before taking office after the Brexit vote last summer, 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.

She repeated calls for international action to combat extremist content online, a message she took to the G7 leaders summit last week.

May also warned there was "far too much tolerance of extremism in our country", promising to review counter-terrorism efforts, including possibly increasing the jail terms handed out in terror cases.

The ruling Conservatives and the main opposition Labour party suspended national campaign events for the day, although local campaigning will continue.

"But violence can never be allowed to disrupt the democratic process, so those campaigns will resume in full tomorrow and the general election will go ahead as planned on Thursday," the prime minister said.

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.

- Echoes of Westminster attack -

Several people said they were ordered by police to stay inside pubs and restaurants as the terror raged outside.

Alex Shellum at the Mudlark pub said a woman had come into the bar "bleeding heavily from the neck", telling the BBC: "It appeared that her throat had been cut."

Italian photographer Gabriele Sciotto, who was watching the football at the Wheatsheaf pub in Borough Market, said he saw three men shot just outside the pub.

In a picture he took, a man wearing combat trousers, with a shaved head and what looked like a belt with canisters attached to it could be seen on the ground with two more bodies behind him.

"In two or five seconds, they shot all the three men down," Sciotto told the BBC.

The attack had harrowing echoes of the attack on Westminster Bridge in March, when British Muslim convert Khalid Masood rammed his car into pedestrians before crashing into the barriers surrounding parliament.

He stabbed a police officer to death before being shot dead by a ministerial bodyguard.