At least three assailants stabbed passers-by at random after smashing into pedestrians in a van, killing seven people in a "terror attack" in a popular nightlife hub in the London Bridge area on Saturday night before being shot dead by armed police.

London police chief, Cressida Dick, speaking to the press outside Scotland Yard on Sunday confirmed that "seven members of the public have died."

Around 48 wounded were taken to five hospitals across the city by the London Ambulance Service (LAS), whereas a number of others have been treated at the scene for minor injuries, the LAS said in a statement after the attack.

France said two of its citizens were injured in the attack, one seriously. The injured also included a police officer who was one of the first responders on the scene and was stabbed in the face and leg.

As they rampaged through the bars around London Bridge, the attackers wore what looked like suicide vests which turned out to be "hoaxes", said Britain's top anti-terrorism officer, Assistant Commissioner Mark Rowley.

The BBC showed a photograph of two possible London attackers shot by police, one of whom had canisters strapped to his body.

The assault began when a van veered off the road and plowed into pedestrians on the bridge.

Witnesses say three men fled the van with large knives and attacked people at bars and restaurants in nearby Borough Market.

Armed police rushed to the scene and shot dead the three male attackers in the Borough Market area near the bridge, as authorities urged Londoners on Twitter to "run, hide, tell" if they were caught up in the violence.

"The suspects had been confronted and shot by police within eight minutes of the first call," Rowley said.

"We believe that six people have died in addition to the three attackers shot dead by police" in what is being treated as a "terrorist incident", Rowley said, adding that while they believe all the attackers were killed, the investigation continues.

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

The Metropolitan Police force has declared the attacks as "terrorist incidents", and police specialists have begun collecting evidence.

Hours after the attack the area remained sealed off and patrolled by armed police and counter-terrorism officers.

Streets around London Bridge and Borough Market, fashionable districts packed with bars and restaurants, would have been busy with people on a Saturday night out.

BBC showed dozens of people, evidently caught up in the attack, being escorted through a police cordon with their hands on their heads.

London's Thames river police said it was working with the lifeboat rescue service to help evacuate people caught up in the attack.

London Mayor Sadiq Khan said Sunday some of those injured in the attacks on the city are in "critical" condition.

Khan said that Londoners should all remain vigilant as the terrorist threat level is severe. But he added: "I'm reassured we are one of the safest global cities in the world."

Khan says the militants want to disrupt Britain's democracy and hamper plans for voting in the general election June 8.

He praised emergency services and said police would make a statement after an emergency cabinet meeting.

British Prime Minister Theresa May said that the "terrible incident in London is being treated as a potential act of terrorism."

May plans to chair an emergency security Cabinet session today to deal with the crisis.

Britain's Conservative Party announced that it is suspending national campaigning ahead of the general election as a result of the attacks.

May's party said there will be no national campaigning Sunday. It is not clear if campaigning will resume in the final days before the vote.

The opposition Labour Party has not yet announced its plans for Sunday.

There was no immediate claim of responsibility for the attack, which comes less than two weeks after a suicide bomber killed 22 people at a pop concert by US singer Ariana Grande in Manchester in northern England.

The incident bore similarities to a March attack on Westminster Bridge, west of London Bridge, in which a man killed five people after driving into a crowd of pedestrians before stabbing a police officer in the grounds of parliament.

The Manchester bombing on May 22 was the deadliest attack in Britain since July 2005, when four British Muslim suicide bombers killed 52 people in coordinated attacks on London's transport network.

Witness accounts: 'They had no clue what they were doing'

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 a terrorist 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 strapped to his chest.

Witnesses described a white van careering into pedestrians near London Bridge and knocking over several people.

"It looked like he was aiming for groups of people. I froze because I didn't know what to do," Mark Roberts, 53, a management consultant, told Reuters.

He saw at least six people on the ground after the van veered on and off the pavement.

"It was horrendous," he said.

A man called Gerard told the BBC, "They were stabbing everyone. They were running up and going 'This is for Allah'." He added that he had seen the assailants stabbing a girl and had tried to confront them.

Another witness called Eric told the BBC he had seen three men come out of a white van after hitting pedestrians and thought they were going to help.

But instead they "started kicking them, punching them and took out knives. It was a rampage really," he said.

Witnesses said people ran into a bar to seek shelter.

"People started running and screaming, and the van crashes into the railing behind. We went toward Borough Market and everyone went inside (the bar). Everyone in the bar started pushing people from the exits," one witness who gave his name as Brian, 32, told Reuters.

Another witness, who declined to be named, his white top covered in blood, described a scene of panic in the bar.

"They hit the emergency alarm. There was a line of people going down to the emergency exit. And then people started screaming coming back up," the 31-year-old said.

"Around the corner there was a guy with a stab wound on his neck... There was a doctor in the pub and she helped him. They put pressure on the stab wound."

BBC radio said witnesses saw people throwing tables and chairs at the perpetrators of the attack to protect themselves.

Gerard Vowls told The Guardian newspaper that he saw a woman being stabbed by three men at the south end of London Bridge. He said he 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 newspaper.

"I want to know if this girl is still alive. I've been walking around for an hour and a half crying my eyes out. I don't know what to do."

Bartender Alex Martinez said he hid in a garbage bin for a half hour when a man stormed the restaurant where he worked, which was nearly full.

"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.

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."

Islamic State call

The militant Islamic State (IS) group earlier on Saturday sent out a call on instant messaging service Telegram urging its followers to launch attacks with trucks, knives and guns against "crusaders" during Ramazan.

Similar attacks, in Berlin, Nice, Brussels and Paris, have been carried out by militants over the past couple of years.

World leaders express solidarity

Prime Minister Nawaz Sharif strongly condemned the terrorist attack, the Press Information Department said Sunday, and said Pakistan stands in solidarity with the people and government of the United Kingdom.

German Chancellor Angela Merkel expressed solidarity with the people of London following the attacks,

Merkel said in a statement Sunday that she learned with "sadness and dismay" of the attacks in the London Bridge area Saturday night.

She said that "today, we are united beyond all borders in horror and sorrow, but also in determination."

She stressed that "in the fight against every form of terrorism, we stand firmly and with determination at Britain's side."

French President Emmanuel Macron, said his nation ─ which has suffered its own wave of terror attacks ─ was "more than ever at Britain's side."

European Commission chief Jean-Claude Juncker said, "Following latest London incidents with horror. Thoughts and prayers are with the victims and their families. Please stay safe."

Canadian President Justin Trudeau said, "Awful news from London tonight. We're monitoring the situation."

Indian Prime Minister Narendra Modi condemned the attacks in London, saying they caused shock and anguish.

In a brief statement after returning from Europe, Modi said that his thoughts are with the families of the deceased and offered prayers for the injured.

United States (US) President Donald Trump took to Twitter to offer US help to Britain. The White House said he had been briefed on the incidents by his national security team.

Trump also argued in favour of his controversial travel ban. One tweet read: "We need to be smart, vigilant and tough. We need the courts to give us back our rights. We need the Travel Ban as an extra level of safety!"