A violent mob attacked two police officers after they asked a teenage girl to pick up a piece of litter.
The incident occurred near to the Woolworths and Burger King in North End, central Croydon, at about 3pm on Wednesday.
It started when the unarmed male officers on patrol saw a teenage girl drop a fast food wrapper from a nearby takeaway and asked her to pick it up. She did so but immediately dropped it on the ground again.
They then separated the girl from a 15-year-old girl with her who was being noisy and unco-operative.
When the officers asked her again to pick it up her friend became aggressive towards them.
Police said she then bit one of the officers on his shoulder or back.
A group of people then gathered around the officers and intimidated and attacked them. It is believed the crowd grew to about 30 people, made up of male and female teenagers and adults, with more people watching.
Police said all suspects were black.
The two officers, aged 34 and 29, suffered from bruising and knee injuries and at one point the 34-year-old was bitten and needed hospital treatment for his injury.
They are both now on sick leave.
The 15-year-old girl was arrested at the scene on suspicion of assault. She has been bailed following further inquiries.
Yesterday morning police also arrested two men, aged 34 and 38 both from South Norwood, on suspicion of assault and violent disorder.