Boris Johnson vows action after terrorist attack on the London Bridge

Prime Minister Boris Johnson vowed Saturday to review Britain's sentencing system after a convicted terrorist released early from prison stabbed two people to death and injured three in a London Bridge attack.

Members of the public have been hailed as heroes for preventing even greater loss of life by tackling Usman Khan -- one armed with a five-foot (1.5-metre) narwhal tusk and another with a fire extinguisher -- before police shot him dead.

Video footage of the confrontation showed Khan, 28, being challenged by a man, reportedly a Polish chef, wielding the tusk -- believed to have been grabbed from the historic hall where the stabbings began -- as another person sprayed him with the extinguisher.

Khan had been conditionally released from jail last December after serving less than half of a 16-year prison sentence for terrorism, and was wearing a fake explosive device.

On Saturday, the Islamic State group released a statement claiming responsibility for the attack.

"The person who carried out the London attack... was a fighter from the Islamic State, and did so in response to calls to target citizens of coalition countries," IS said, referring to a multi-country alliance against the group.

The incident comes two years after Islamist extremists in a van ploughed into pedestrians on London Bridge before attacking people at random with knives in nearby Borough Market.

On that occasion, eight people were killed and 48 wounded before the three attackers, who were wearing fake suicide devices, were shot dead by police.

Metropolitan Police Assistant Commissioner Neil Basu said five people had been stabbed in all inside Fishmonger's Hall before members of the public pursued the attacker onto London Bridge. The three survivors remain in hospital.

Basu added that Khan had been released under "an extensive list of licence conditions" with which he had previously been complying.

Police on Saturday searched two properties in Stoke-on-Trent, Khan's home city, and Stafford in central England.

The latest attack came less than two weeks before Britain's general election, and politicians temporarily suspended campaigning.

"It does not make sense for us as a society to be putting people who have been convicted of terrorist offences... out on early release," Johnson, who became Tory leader in July, said as he visited the scene.

"We argue that people should serve the tariff, serve the term, of which they are sentenced," the prime minister added, noting the Conservatives' manifesto called for a tougher sentencing regime.

'Bundle him to the ground'

Johnson spoke hours before the first victim of the attack was named as Jack Merritt, a course coordinator at Cambridge University's criminology institute, according to media reports.

The institute hosted a prisoner rehabilitation event at Fishmonger's Hall, a historic building on the north side of the bridge. Khan attended the event reportedly armed with two knives and the fake suicide vest.

As the confrontation moved from inside the hall to the pavement outside, a throng of people could be seen in videos grappling with Khan.

They reportedly included a convicted killer on day-release from prison and other ex-offenders also attending the criminology gathering.

Tour guide Stevie Hurst told BBC radio: "Everyone was just on top of him trying to bundle him to the ground.

"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," he said.

A British Transport Police officer in a suit and tie who also intervened was seen carrying a large knife away.

"As we saw the worst of human kind, we saw the very best of human spirit and London," Met Commissioner Cressida Dick said Saturday as she visited London Bridge.

'Enduring thanks'

Khan, a British national, was handed an indeterminate sentence for the protection of the public in 2012, with at least eight years in prison.

He was part of an eight-man network inspired by Al-Qaeda who had plotted to bomb targets including the London Stock Exchange, and planned to take part in "terrorist training" in Pakistan.

But his sentence was quashed by the Court of Appeal in April 2013 and he received a new 21-year term, comprising a custodial sentence of 16 years and five years on conditional release.

Inmates are usually released half-way through this type of determinate sentence -- and time spent in custody before trial may have been taken into account.

The Parole Board said it had no involvement in his release and that it appeared to have happened automatically as required by law.

Johnson, who took over as prime minister in July, said the cases of other convicted terrorists released early were under urgent review.

Queen Elizabeth II expressed her "enduring thanks" to the "brave individuals who put their own lives at risk to selflessly help and protect others".