Gemma Daly, 31, from Whitstable, said she heard screams and “a big bang”, which she believes was a gunshot.
He said he heard police sirens immediately, before emergency services began arriving soon after.
“Police were attending to a man on the side of the bridge,” he said, adding that he was “clearly injured.”
He added that “a lot of people” were working on the man and had “taken his clothes off” before putting him on a stretcher.
Footage from the scene showed emergency vehicles, including police cars and an ambulance, stopped in the middle of the road.
A spokesman for the London Ambulance Service said a number of resources, including medics from the London Air Ambulance, had been sent to the scene.
“We treated a man at the scene and took him as a priority to a major trauma centre,” they said.
The Met said significant cordons remained in place on Friday evening and urged the public to avoid the area.
Police said the Directorate of Professional Standards and the Independent Office for Police Conduct have been informed.