A gang of up to 30 people attacked two policemen - biting one of them - in south London after the officers asked a teenage girl to pick up some litter she had dropped, Scotland Yard said on Friday.

The two officers, aged 29 and 24, were on duty in central Croydon on Wednesday afternoon when they saw the girl drop a piece of litter. They asked her to pick it up, which she did, but then proceeded to immediately drop it on the ground again.

When she was asked to pick it up again, a friend with her became aggressive and a group of teenagers then surrounded the officers and began to attack them. Police said a crowd of up to 30 people became involved.

The older officer was bitten during the attack and needed hospital treatment while both suffered from bruising and knee injuries.

"We have heard from several retailers in the area who were horrified at the level of violence used by the crowd against the officers," said Inspector Simon Ellingham.

Police said a 15-year-old girl was arrested at the scene on suspicion of assault. Two men, aged 34 and 38 and from South Norwood in south London, were arrested the following day on suspicion of assault and violent disorder.