A suspected terrorist was wrestled to the ground and shot dead by police in front of crowds on London Bridge on Friday after a stabbing in which “a number” of people were injured.
The dead man was wearing an explosive device that later turned out to be a hoax, Britain’s top anti-terrorism police officer said.
The condition of those injured remains unclear.
It comes only three weeks after Britain lowered its national terrorism threat level to “substantial” from “severe”, its lowest level since 2014.
As counter-terrorism police began an investigation, Prime Minister Boris Johnson said that “anybody involved in this crime and these attacks will be hunted down and will be brought to justice.”
Britain “will not be cowed, divided or intimidated” by such attacks, he added.
London Mayor Sadiq Khan praised “the breathtaking heroism” of members of the public who thwarted the suspect and “ran towards danger not knowing what confronted them”.
He said those who intervened didn’t know that the device worn by the suspect was fake.
“They really are the best of us, another example of the bravery and heroism of ordinary Londoners,” Khan added.
Officers from the Metropolitan Police and City of London Police were called to the busy London Bridge area — the scene of another deadly terror attack in 2017 — amid reports of a stabbing.
Neil Basu, Assistant Commissioner for Specialist Operations at the Metropolitan Police, said: “A male suspect was shot by specialist armed officers from the City of London Police and I can confirm that this suspect died at the scene.
“A number of other people received injuries during this incident. As soon as we can provide further updates on their condition, we will.
“As you would expect, due to the nature of the incident, we responded as though it was terrorist-related. I am now in a position to confirm that it has been declared a terrorist incident.”
Details of what first happened were not immediately clear, but a video posted on Twitter showed a group of people wrestling the suspect on the ground before armed police arrived and two gunshots were heard.
London Bridge station, one of Britain’s busiest, was closed and evacuated.
Nearby buildings, including the nearby offices of News UK, home to The Times and Wall Street Journal, were placed on lockdown.
London Bridge was the scene of an attack in June 2017 when three militants drove a van into pedestrians and then attacked people in the surrounding area, killing eight people.