A convicted terrorist wearing a fake explosive vest stabbed several people in London, killing two before being tackled by members of the public and then shot dead by officers on London Bridge.

Key points: The attacker has been identified as 28-year-old Usman Khan

The attacker has been identified as 28-year-old Usman Khan Police say he had served time for a terrorism-related offence

Police say he had served time for a terrorism-related offence He was reportedly attending a criminology conference near London Bridge

Police said one man and one woman had died and three injured people were being treated in hospitals after the attack, which unfolded near the site of a 2017 terror attack that left 11 people, including two Australians, dead.

Police identified the attacker as 28-year-old Usman Khan. They said he had been convicted of a terrorism offence in 2012 and released in December 2018. They said he was wearing a "hoax explosive device" during the attack.

In a statement, the Parole Board said it had no involvement with the release of the attacker, who they said was "released automatically on licence (as required by law), without ever being referred to the Board".



Usman Khan was convicted of a terror offence in 2012. ( Supplied )

British media reported that Khan had recently been freed from prison on condition that he wear a tracking device. The Times of London said he stabbed people at a criminology conference he was attending in Fishmongers' Hall, near where he was shot dead.

British police will look at the conditions placed on people like the London attacker, a junior minister said.

"There are conditions that are put on people in this situation and one of the things the police will be looking at is those conditions as part of that investigation," junior interior minister Brandon Lewis told BBC radio.

Metropolitan Police Assistant Commissioner Neil Basu said police were not actively seeking anyone else in relation to the attack, but officers were carrying out searches at a Staffordshire address on Friday night.

Video posted to social media showed several members of the public wrestling with Khan on London Bridge before armed police officers arrived.

The armed officers separated the group before firing two shots at Khan as he lay on the ground.

A witness at Fishmongers' Hall said she saw a man take a 1.5-metre narwhal tusk from a wall inside the historic building to confront the attacker with.

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London Mayor Sadiq Khan praised members of the public who ran toward the attacker and wrestled him to the ground for their "breathtaking heroism".

The Times reported the attacker was a guest at a conference being held by University of Cambridge-based prison-education organisation Learning Together.

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The report said he had threatened to blow up the venue before stabbing several people.

The University of Cambridge said it was "gravely concerned" about students, staff and alumni who might have been caught up in the incident.

Appealing for anyone who witnessed the incident or had video footage of the attack to come forward, Metropolitan Police Chief Cressida Dick said police were working to identify the two victims and notify their families.

Police asked members of the public to keep clear. ( AP: Dominic Lipinski )

Australian Prime Minister Scott Morrison described the attack, and another stabbing in The Hague, where multiple people were injured, as "terrible."

"We are not aware of any Australians being caught up in these attacks at this stage, but we are in close contact with local authorities," he said on Twitter.

City of London Police had a number of officers responding to the incident. ( AP: Dominic Lipinski )

BBC News reporter John McManus was walking across London Bridge when he saw part of the incident unfold.

"There appeared to be a fight going on on the other side of the bridge, with several men attacking one man," he told the BBC.

"Police then quickly arrived, including armed police, and then a number of shots were fired at this man."

Police said they had shot and killed the attacker. ( AP: Dominic Lipinski via PA )

British Prime Minister Boris Johnson hailed emergency services and members of the public who intervened "to protect the lives of others".

"They represent the very best of our country and I thank them on behalf of all of our country," he said.

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He also said anyone involved in the attack would be "hunted down and will be brought to justice."

"I think the message that we send to them and anyone associated with this type of attack is one that will be familiar, and that is that this country will never be cowed or divided or intimidated by this sort of attack," he said.

The Conservatives and Labour both said they would suspend their election campaigns in London in the wake of the attack.

Meanwhile, police in the Netherlands launched a hunt for a man who stabbed three people at a busy shopping district in The Hague on Friday night (local time).

A police spokesperson said the three victims, who were all youths, were being treated for injuries in hospital.

Police have not reported a motive for that attack.

A police officer moves an uninvolved person away from a cordon after the stabbing on London Bridge. ( AP: Dominic Lipinski/PA )

ABC/wires