A man wearing a suspected hoax explosive device knifed two people to death in a terror attack on Friday before being shot dead by police on London Bridge.
Three more people were injured in the stabbing spree which revived memories of a three-man attack on London Bridge two years ago that killed eight people.
Footage on Twitter filmed by eyewitnesses showed a scrum of people tackling the suspect on the ground before specialist armed officers arrived and fired as he made to get up.
London Mayor Sadiq Khan praised those onlookers who risked their own safety to try to disarm the suspect before the police arrived.
Police said they were called at 1:58pm (1358 GMT) and confronted the man by 2:03pm, shooting him dead.
The attack came a few hours before a number of people were wounded in a stabbing in one of the main shopping streets in the centre of the Dutch city of The Hague.
The two attacks 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 incident in London came less than two weeks before a general election at which Prime Minister Boris Johnson hopes to win a majority to enable him to take Britain out of the European Union.
The premier cut short a visit to his constituency to return to central London for a briefing by the police and a meeting of Britain's emergencies committee.
Johnson praised the emergency services and the public for their response, adding: "This country will never be cowed or divided or intimidated by this sort of attack."
Both Johnson's Conservatives and the main opposition Labour party have both pledged to put at least 20,000 more police officers on the streets.
But Brexit has raised questions about the extent of continued cooperation with Europe on security and intelligence matters.
Metropolitan Police chief Cressida Dick said she was "deeply saddened and angered that our city has again been targeted by terrorism".
She said the attack began at the Fishmongers' Hall on the north side of the bridge.
Neil Basu, the head of UK counter-terrorism policing, said the suspect died at the scene and a terrorist incident had been declared.
"We retain an open mind as to any motive," he told reporters.
"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 carrying a large knife away from the group.
Three armed officers then appeared on the scene and surrounded the scrum. One officer pulled one last man away a split-second before two shots appeared to be fired.
"Everyone was just on top of him trying to bundle him to the ground," said Stevie Hurst, who ran from his car to the scene.
"I saw that the knife was still in his hand so I just put a foot in to try and kick him in the head: we were trying to do as much as we could to try and dislodge the knife," he told BBC radio.
"The guys that were there were just amazing. Absolutely incredible people. Heroes beyond belief."
The bridge was closed off, with evacuated buses at a standstill. Forensics officers were at the scene.
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.
Threat level downgraded
On November 4, Britain downgraded its terrorism threat level from "severe", the second-highest of five levels, to "substantial" -- its lowest rating in more than five years.
The downgrading meant a terror attack was considered "likely" rather than "highly likely".
The 2017 London Bridge attack involved Islamist extremists in a van who ploughed into pedestrians on the 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.