Police investigating the van and knife attack in central London have arrested 12 people in Barking, in the east of the capital.

Armed officers swooped on a block of flats, where one of the three killers was believed to have lived.

Several women were among those taken away from the property by police.

The arrests come after three attackers drove a van into pedestrians on London Bridge just after 10pm on Saturday before stabbing others in nearby Borough Market in an attack that killed seven people and injured 48.

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At Borough Market, the terrorists, reportedly armed with foot-long knives, jumped out of the white van and stabbed people, including a British Transport Police officer who has serious but not life-threatening injuries to his face, head and leg.

Within eight minutes, armed police confronted three male attackers, shooting them dead.

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Metropolitan Police commissioner Cressida Dick condemned the atrocity as "appalling" and "ghastly". She said police believe all attackers had been "neutralised".

Referring to the speed with which officers were able to react to the incident she said: "We were already at a very high level of alertness.

"Severe means that an attack is highly likely and the threat level was at severe. We were prepared potentially for an incident as we have been for some considerable time."

Appealing to Londoners and visitors "to remain calm, be very vigilant", she also warned: "The last thing we need is people overreacting or take out there frustrations on other people in other communities".

Asking people to contact police if they see anything out of the ordinary, even if insignificant, she said where possible "people should carry on with their normal lives".

Image: New Zealander Oliver Dowling is reported to have been stabbed in the face, neck and stomach

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The injured were taken to five hospitals across London, according to the city's ambulance service. More were treated at the scene for less serious injuries.

Victims include New Zealander Oliver Dowling, 32, who has reportedly been left in a coma after being stabbed in the face, neck and stomach.

His girlfriend Marie Bondeville has also been injured in the attack.

Australian woman, Candice Hedge, 34, has undergone emergency surgery after being stabbed in the throat while eating dinner with her boyfriend.

Sunday Express business editor Geoff Ho was also slashed in the throat after stepping in to help a lone bouncer being targeted by the attackers.

An off-duty Metropolitan police officer and British Transport police officer - who was one of the first responders on the scene - were also caught up in the attack.

Both men are understood to be in a serious but stable condition.

A Canadian citizen has also been "directly affected" by the attack.

France's President Macron said two of the victims were French citizens, with one in a serious condition.

Image: A man on the ground with canisters attached to his chest. Pic: Gabriele Sciotto

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The Premier Bankside hotel was evacuated and three hospitals - Guy's, St Thomas' and Evelina London Children's - went into lockdown "to keep patients, relatives and staff safe".

Image: People were told by police to leave the area with their hands up

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In response to the attack,Theresa May has said there is far too much tolerance of extremism in the UK and it is time to be more robust in tackling it.

Speaking in Downing Street following the van and knife attack, the Prime Minister said it was time to say "enough is enough".

National campaigning in the General Election has been suspended by the Conservatives and Labour in the wake of the attack, with the PM likely to come under pressure to cancel or postpone Thursday's poll.

But London Mayor, Sadiq Khan, who described the atrocity as a "barbaric act", has insisted he is not in favour of delaying the election.

He said: "One of the things we can do to show we are not cowed is to vote in the General Election on Thursday."

Image: Police attend to an incident on London Bridge

Spectator journalist Will Heaven was at London Bridge shortly after the incident and wrote on Twitter that he had seen casualties - one on the pavement and one on the edge of the road.

He told Sky News: "It was about 10.15pm. I was in the back of an Uber cab driving over London Bridge...there was somebody down on the pavement with a small crowd around them clearly very concerned.

"It looked like someone had collapsed. We drove a bit further and noticed there was another person on the right hand side of the bridge who was also not on the pavement but in the road itself.

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"The penny dropped something quite serious was happening. Suddenly the traffic came to standstill. The driver said something bad is happening here.

"We could hear sirens coming and the suddenly ahead of us we noticed another casualty in the road."

He added that "it did not look like an accident", adding it was clear that police "thought they were responding to something far more serious than a road traffic accident".

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A witness who gave his name as Jamie was in a restaurant near London Bridge. He said: "We heard a fight and everyone got up and everyone rushed out of the restaurant and we heard a massive, massive bang.

"Then we hid under the table and people came into the restaurant and knocked a bunch of stuff over, like the till.

"And then we ran into the restaurant into the kitchen, where there was a bunch of other people and a guy had been stabbed and he was cut and he was bleeding quite a lot."

Image: Emergency services attend to an incident near London Bridge in London

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A female companion said: "We were in the restaurant and we just saw three guys come into the restaurant, stabbed someone in the face and someone in the stomach.

"One of them had a big knife, then he came in and walked around the restaurant, I guess they just kind of stabbed anyone that they saw and knocked things on the ground and then we just hid."

Police asked people to avoid the areas affected, with London Bridge being closed in both directions. London Bridge, Bank, Vauxhall and Borough stations were closed, although Bank and Vauxhall have since reopened.

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The attacks come less than two weeks after a suicide bomber killed 22 people at a concert in Manchester.

Ariana Grande, who had been performing that night, tweeted: "Praying for London".

:: Anyone concerned about friends or relatives who may have been caught up in the attacks can call the police casualty bureau on 0800 096 1233 or 020 7158 0197.

:: London terror attack - Watch a special programme on Sky News presented by Dermot Murnaghan at 8pm tonight.