A fairground reveller was arrested on suspicion of attempted murder after stabbing a police officer in the neck with a bottle.
The officer, aged in his thirties, chased the suspect and arrested him in a nearby street after he was assaulted at a funfair in Carshalton, south London.
Detective Sergeant Nick Burrows, who is leading the inquiry, said the officer was attacked without warning from behind.
He said the officer suffered a minor wound to his neck and was protected from more serious injury by his stab-proof vest.
Mr Burrows said: "This was a random attack on an officer. His Metvest prevented him from suffering a more serious injury."
The attack took place in the recreation ground in Wrythe Lane, Carshalton, south London, at about 8.30pm on Sunday.
The victim was an officer based at Sutton's Safer Neighbourhoods Team, tasked with dealing with local disorder and nuisance.
A Metropolitan Police spokesman said: "We are appealing for witnesses after a police officer was attacked with a bottle in the neck at a fair
"The officer from the Safer Neighbourhoods Team was talking to members of the public when he was attacked from behind by a man wielding a bottle.
"He received a cut to his neck but did not require hospital treatment."
The suspect remained in custody at a south London police station today.
* Anyone who witnessed the incident should contact Sutton Police Station on 0208 649 0743.