A man with a hoax suspect device strapped to him has been killed by police in what's been described as a terrorist incident in central London. A number of people have been injured.

Police have not commented further on the motive for the incident near London Bridge, but Assistant Commissioner Neil Basu told a press conference on Friday afternoon that they were enhancing patrols in the area and across the city in response.

Video filmed at the scene by an eyewitness showed a man being wrestled to the ground by members of the public. The incident comes more than two years after eight people were killed and dozens more injured in a terror attack at London Bridge.

Lloyd Griffiths, 35, was on a bus on the bridge on Friday as the incident unfolded. He told CNN he saw an altercation as several people tried to wrestle a man with a knife before police intervened.

"I saw a shine on a knife or metal blade, it was startling. It was large blade, it wasn't small, and then I was locked on the bus, people tried to tackle to man trying to fight him, ordinary people jumping out of the car, trying to fight him," Griffiths told CNN.

"Then police ran over with guns, screaming," before shooting the man, Griffiths added. A large number of emergency vehicles quickly arrived on the scene, with many people evacuated from the area.

Prime Minister Boris Johnson said he is "being kept updated on the incident at London Bridge and want to thank the police and all emergency services for their immediate response."

Other videos showed people evacuating pubs near the bridge, and reports suggested the incident broke out in a premises before moving towards the bridge. London Bridge is a busy area of the city which houses several restaurants, bars and businesses. A terror attack on the bridge in 2017 killed several people.

Guardian columnist Owen Jones told CNN: "People were running .... they'd been told to urgently evacuate the area."

"I have friends who have been evacuated from their places of work ... others (are) in lockdown," added Jones, who said police cordoned off a "sizeable" area around the bridge.

London Bridge train station, which serves the area, is closed and trains are not stopping, British Transport Police said. London mayor Sadiq Khan said he is in "close contact" with police.

Jinnat Ui Hasan told CNN he was in a meeting in a building near the incident when he heard "more than five" gunshots.

Elsewhere, Stevie Beer, 26, told CNN that her place of work which overlooks the bridge was in lockdown. "A colleague out for lunch messaged to say there had been an incident on London Bridge and he was unable to cross back over the river. I went straight to the window and saw hundreds of people running off the bridge down Tooley Street," Beer told CNN.

"People looked so panicked. It was haunting. There were at least half a dozen police cars and halted buses. Our building is now on lock down but we've not been given any concrete information," she added.

This is a breaking story. More to follow.

CNN's Bianca Britton and Gianluca Mezzofiore contributed reporting.