Three men have been arrested after a police officer was stabbed, Scotland Yard said.
The officer, whose injuries are not thought to be life threatening, was stabbed in Bute Road, Waddon, south London, at about 9.45pm last night.
The three men are being held in separate south London police stations, a Metropolitan Police spokesman said.
The male officer has been treated for stab wounds and is in a serious condition in a south London hospital, police said.
He was part of a "proactive plain clothes patrol" by officers based at Croydon borough.
"The officer was on patrol with colleagues and had stopped a group of males in Bute Road when the suspect, one of the males stopped, stabbed him," a police spokesman said.