A 14-year-old girl has been arrested on suspicion of murdering a 15-year-old boy.
Officers were called to reports of a disturbance on the Loughborough Estate in Brixton, south London, on Thursday night.
A Scotland Yard spokesman said officers, London Ambulance Service and London's air ambulance attended after receiving the emergency call at 9.24pm and found the boy with serious stab wounds.
He was pronounced dead at the scene.
Police believe they know the victim's identity but await formal identification. Next of kin have been informed.
A post-mortem examination will take place at Greenwich mortuary on Friday.
Lambeth-based detectives, together with officers from the homicide and serious crime command, are investigating.
An incident room was opened under Detective Chief Inspector Charles King. The incident is not believed to be gang-related.
Mr King said: "We would appeal for anyone who has any information or witnesses to come forward as soon as possible."