Gemma Daly, 31, of Whitstable, said she heard screams and ” big explosion,” which she thought was a gunshot.
She said she immediately heard police sirens before emergency services began arriving soon after.
“The police came to the man on in side of bridge,” she said, adding that he was “obviously injured.”
She is added is that a lot of people”Worked on man and had”removed his clothes” before they put him in on stretcher.
Pictures of the scene showed emergency vehicles, including police cars and ambulances, stopped in middle of road.
A spokesman for the London Ambulance Service said they sent several specialists to the scene, including medics from the London Ambulance Service.
“We assisted the man at the scene and took him as a priority to a major trauma center,” they said.
The Met said that significant cordons remain. in place on Friday night and called public to avoid in area.
The police said that the of Professional standards and an independent office for The conduct of the police has been informed.