A man wearing a suspected hoax explosive device was shot dead by armed officers on London Bridge on Friday after what police said was a terrorist stabbing spree, reviving memories of another horrific attack two years ago that killed eight.

Footage on Twitter filmed by eyewitnesses showed a scrum of people tackling a man on the ground before specialist armed officers arrived and shot him dead as he tried to get up.

It is not known how many people were injured in the stabbing spree before the shooting but London Mayor Sadiq Khan said several people were "seriously" hurt.

Khan said members of the public risked their own safety to try to disarm the suspect. "They are the best of us," he told reporters.

The events came less than two weeks before an election at which Prime Minister Boris Johnson hopes to win a majority to enable him to take Britain out of the European Union.

The prime minister cut short a visit to his constituency in northwest London to return to his Downing Street office for a briefing by the police.

Both Johnson's Conservatives and the main opposition Labour party have both pledged to put at least 20,000 more officers on the streets.

But Britain's impending Brexit has raised questions about the extent of continued cooperation with Europe on security and intelligence matters.

New European Council President Charles Michel said he had been "very saddened to hear the dramatic reports" from London, adding: "The fight against terrorism is a common fight."

Johnson praised the emergency services for their response, adding: "This country will never be cowed or divided or intimidated by this sort of attack.

"Our values, our British values will prevail."

The attack happened on so-called "Black Friday", one of the busiest shopping days of the year in the run-up to Christmas, bringing more shoppers onto the streets.

The Metropolitan Police said it was called at 1:58 pm (1358 GMT) to a stabbing a premises near the bridge, which links The City financial district to the south bank of the River Thames.

"A male suspect was shot by specialist armed officers," Neil Basu, the head of UK counter-terrorism policing, told reporters outside New Scotland Yard police headquarters.

"This suspect died at the scene. It has been declared a terrorist incident. We retain an open mind as to any motive.

"We believe a device that was strapped to the body of the suspect was a hoax explosive device."

Several people were seen in video footage apparently surrounding the man on the ground.

One man, wearing a suit and tie, was seen retreating from the scene, with what appeared to be a large knife in his right hand.

Three armed officers then appeared on the scene and surrounded the scrum. One officer pulled a man away before two shots appeared to be fired.

An eyewitness told reporters: "He was on the ground with a knife near him, and a bag near him and then they surrounded him and armed police shot him."

Another told the BBC she and other people in the area rushed into a nearby restaurant and were told by staff to take cover under tables.

The bridge was closed off, with evacuated buses at a standstill, and a white truck straddling the carriageways. Forensics officers were at the scene.

Johnson thanked the police and all emergency services "for their immediate response".

A White House spokesman said US President Donald Trump had been briefed on the attack and was monitoring the situation.

Trump, who has previously criticised Mayor Khan and stabbings in the British capital, is due to visit next week for a NATO summit.

Britain downgraded its terrorism threat level from "severe", the second-highest of five levels, to "substantial" on November 4, its lowest rating in more than five years.

British officials consider a terrorist attack "likely" when the threat level is substantial, and "highly likely" when it is severe.

The Joint Terrorism Analysis Centre, an organisation made up of Government departments and other public bodies, makes the decision after weighing up intelligence and information on known terror groups.

The 2017 attack involved Islamist extremists in a van who ploughed into pedestrians on London Bridge before attacking people at random with knives in nearby Borough Market.

Eight people were killed and 48 others injured. The three attackers, who were wearing fake suicide devices, were shot dead by police.