Members of the public who wrestled a knife-wielding terrorist to the ground on London Bridge have been praised as heroes.

Key points: The attacker was tackled to the ground before being shot dead by police

The attacker was tackled to the ground before being shot dead by police London Mayor Sadiq Khan praised the bystanders as "the best of us"

London Mayor Sadiq Khan praised the bystanders as "the best of us" One man reportedly grabbed a narwhal tusk to use as a weapon

Convicted terrorist Usman Khan, who was only released from jail last year, killed two people and injured three more before being overpowered by bystanders and shot by police on Friday afternoon (local time).

Video posted on social media showed two men struggling on the bridge before police pulled a man in civilian clothes off a black-clad man on the ground. Gunshots were then heard.

Another man in a suit and overcoat, holding a long knife apparently taken from the attacker, was seen walking away.

The man in the khaki pants can be seen holding what appears to be a narwhal tusk, which he used to help subdue the attacker. ( Supplied: Twitter )

One witness said a man who was with her at Fishmongers' Hall, a historic building near London Bridge, took a 1.5-metre-long narwhal tusk from the wall and went out to confront the attacker, who was wearing a fake explosive vest.

"We were trying to help victims inside but that man's a hero," Amy Coop tweeted.

Bystanders flee during the attack. ( AP: Dominic Lipinski )

"What's remarkable about the images we've seen is the breathtaking heroism of members of the public who literally ran towards danger, not knowing what confronted them," London Mayor Sadiq Khan said at a press conference.

"They really are the best of us, another example of the bravery and heroism of ordinary Londoners running towards danger, risking their own personal safety to try and save others.

"I want to say thank you to them on behalf of all Londoners."

'Turn off your engine, get off and run'

Tour guides Thomas Gray and Stevie Hurst said they helped pin Khan down to stop him from killing or injuring more people.

"We were trying to do as much as we could to try and dislodge the knife from his hand so he wouldn't harm anyone else," Mr Gray said.

"When we got there he was wielding two knives, one was duct-taped to his hand. So all I could do after the guys had held him down and were pinning him to the ground, I tried to stamp as hard as I could on his wrist to try and release the knife.

"Someone kicked the knife away, somewhere northbound up London Bridge, and then after that the police armed response were really quick, got there almost instantaneously, and at that point we were told he had a bomb vest so we cleared house and got out the way."

Mr Hurst said he kicked Khan in the head while he was down.

"Everyone was just on top of him trying to bundle him to the ground. We saw that the knife was still in his hand," Mr Hurst told the BBC.

"I just put a foot in to try and kick him in the head."

A police officer moves a bystander away from a cordon during the incident. ( AP: Dominic Lipinski/PA )

Bus driver Mustafa Salih, 62, was travelling from Borough High Street towards London Bridge where he saw emergency vehicles and the police cordon.

He told BBC London: "A police officer came up to me and said turn off your engine, get off and run.

"I looked up and I could see a crowd of people coming towards me.

"One woman was crying. I ran back down to Borough High Street. It was all very scary as we did not know what was happening."

Prime Minister Boris Johnson also praised the bravery of those who intervened and said that the UK would "never be cowed or divided or intimidated by this sort of attack".

Members of the public also expressed praise for the bravery of those involved.

"Absolutely stunned at the bravery of those on London Bridge who took the attacker down and then disarmed him — whilst he was wearing what seemed to be an explosive vest," tweeted Liam Young.

"Those who attack this city with an aim to divide us will never succeed."

City of London police locked down the scene after shooting the suspect. ( AP: Dominic Lipinski )

AP/ABC