Shocking footage shows moment hero grabs knife from London Bridge attacker

Stills from a video shared on Twitter of police detaining a man on London Bridge. Photo / Supplied

Brave bystanders bundled a knifeman to the ground on London Bridge before disarming him and fleeing as police open fired.

The shocking moments were captured on video from the back of a double-decker bus on the bridge and shows the group of people tackling a man on the bridge.

The Sun reports that a man in a suit got a weapon away from the knifeman then walked away from the scene.

Police then urges the bystanders to stand back before a firearms officer shoots down the knifeman.

The Met Police have confirmed they are treating the incident as a "terror-related" attack.

Two people are now confirmed to have died with a number of others injured.

London Bridge was put on lockdown after the shots were fired and several witnesses had to run away in terror.

Guy Lawrence, 48, told the Times he saw a man running out of a building, with two kitchen knives with 20cm blades.

He said a member of the public was seen running after the suspect, one armed with a 6ft stick and another with a fire extinguisher.

"I saw all traffic stopped in front of me. I heard shooting — there was an attack. And I saw a guy on the floor. I heard two shots: bang, bang," he said.

"Police were running towards us and they told us to get out and run."

Another witness said she heard two gunshots before running away from the scene.

"I was below the bridge about 30m to one side of it," Reem Yousef told Sky News.

Stills from a video shared on Twitter of police detaining a man on London Bridge. Photo / Supplied

"I heard shouting and I looked up and it appeared to be about four or five people in a fight.

"Then I heard someone shouting to get past, which was very difficult because police cars were flooding the scene and people were running obviously away from the scene.

"People running down Gray's Church street. I heard sirens very quickly after the gunshots so I think the emergency response was fairly quick.

"And from my perspective I was running away from the scene.

"What was going through my mind was just 'run'. I didn't know what direction the gunshots were going to be coming from so I was just trying to flee the scene and get as far away as I could."