A suspected drug dealer on a bike tried to escape a policeman during a chase through Parsons Green only to crash to the pavement when a member of the public stepped in front of him.
The man is alleged to have been hanging around outside The White Horse Pub throughout Saturday offering drugs to revelers at an American beer festival and scores of people who attended the annual fair on the green.
Police spotted the man at about 7pm and launched a chase when the suspect clambered onto his bike and rode off. But he came down on a pavement in Parsons Green Lane when a passer-by saw what was happening and stepped in front of his path, causing him to veer out of the way and crash to the ground.
Scores of people took to Twitter to find out what was going on after the area was sealed off and awash with emergency vehicles.
Blogger Sophie Carr, who had been drinking in The White Horse, said there had initially been cheers when the chase began.
"When the policeman started running after the guy, people were laughing and didn't think it was that serious but then all the tape went up and there was concern, people were wondering what had happened," she said. "I went over there and it looked pretty serious, the guy was not moving at all."
A spokeswoman for the police said: "We were called at about 7.05pm on Saturday, July 6 to reports of a man dealing drugs in a pub in Parsons Green. Officers attended and upon seeing police a man attempted to make off from them on a bike and police ran after him. Seeing this, a member of the public stepped in front of the suspect and in avoiding them he fell from his bike and injured his head.
"The man, aged 32, was taken to a central London hospital for treatment and was later discharged and taken into custody after being arrested on suspicion of offering to sell a controlled substance."
The suspect has been released on police bail until a date in August. The Independent Police Complaints Commission has been informed of the incident as is procedure after an accident involving officers.