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The BBC reported that two members of the public were killed in what police have called a terrorist attack. Reuters was not immediately able to confirm the deaths.
The attacker went on the rampage just before 2pm (local time), attacking people at a premises near London Bridge in the heart of the city's financial district - the scene of a deadly attack by Islamist militants two years ago.
Half a dozen bystanders tackled the suspect to the ground and grabbed his knife. A video posted on Twitter showed police dragging one man off the suspect before an officer took careful aim. Two shots rang out. The man stopped moving.
Prime Minister Boris Johnson, who has called a snap election for December 12 and is due to host NATO leaders including U.S. President Donald Trump next week, praised those who took on the man for their courage and said Britain would never be cowed.
"I ... want to pay tribute to the extraordinary bravery of those members of the public who physically intervened to protect the lives of others," Johnson told reporters in Downing Street.
"For me they represent the very best of our country and I thank them on behalf of all of our country," Johnson said. "This country will never be cowed, or divided, or intimidated by this sort of attack."
He said the incident was now thought to have been contained and vowed that anyone else involved would be hunted down.
Britain's top counter-terrorism officer, Neil Basu, said specialist armed officers from the City of London police shot the suspect, who died at the scene.
"A number of other people received injuries during this incident," Basu said. "We believe a device that was strapped to the body of the suspect was a hoax explosive device."
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Attacker named, previously convicted of terrorism offences
British police named the man who stabbed two people to death in London on Friday, in what the authorities called a terrorist attack, as 28-year-old Usman Khan, who had been convicted of terrorism offences and was released from prison last year.
"This individual was known to authorities, having been convicted in 2012 for terrorism offences," Britain's top counter-terrorism police officer, Neil Basu, said in a statement.
"He was released from prison in December 2018 on licence and clearly, a key line of enquiry now is to establish how he came to carry out this attack," Basu said.
A person who is released on licence is subject to conditions for the duration of their sentence after leaving prison. The Times newspaper reported that Khan had agreed to wear an electronic tag.
The attacker went on the rampage just before 2pm, targeting people at Fishmongers' Hall near London Bridge in the heart of the city's financial district - the scene of a deadly attack by Islamist militants two years ago.
In addition to the man and the woman who were killed, a man and two women were injured and remain in hospital, Basu said.
Just before news broke of the suspect's previous conviction, Prime Minister Boris Johnson, who is seeking re-election on Dec. 12, said criminals must be made to serve their sentences.
"It is a mistake to allow serious and violent criminals to come out of prison early, and it is very important that we get out of that habit and that we enforce the appropriate sentences for dangerous criminals, especially for terrorists," he said.
Johnson leads opposition leader Jeremy Corbyn, according to opinion polls.
During the 2017 election campaign, London Bridge was the scene of an attack when three militants drove a van into pedestrians and then attacked people in the surrounding area, killing eight and injuring at least 48. The attack focused attention on cuts to policing since the governing Conservatives took power in 2010.
"We owe a deep debt of gratitude to our police and emergency services, and the brave members of the public who put themselves in harm’s way to protect others," Corbyn said late on Friday.