Gemma Daly, 31, from Whitstable, said she heard screams and “a big bang”, which she believes was a gunshot.
She said she heard police sirens immediately, before emergency services arrived shortly after.
“Police were on a man on the side of the bridge,” she said, adding that he was “clearly injured”.
She added that “a lot of people” were working on the man and had “taken off his clothes” before he was put on a stretcher.
Photos of the scene showed emergency services, including police cars and an ambulance, stopping in the middle of the road.
A spokesman for the London Ambulance Service said they had sent multiple agents, including London Air Ambulance medics to the scene.
“We treated a man at the scene and took him to a major trauma center as a priority,” they said.
The Met said significant deposits remained in place Friday night and urged the public to avoid the area.
Police said the Professional Standards Directorate and TBEN Police Conduct Bureau have been notified.
