A 16-year-old boy was stabbed repeatedly outside a party attended by up to 100 people in south London.
The teenager was left seriously injured in hospital after he was attacked outside a party at a community centre in Vauxhall.
Scotland Yard said police were called to the scene in Glasshouse Walk at about 11.30pm on Monday.
Paramedics also attended and the boy was rushed to hospital where he remains in a stable condition. His injuries are not life-threatening, police said.
Police said the victim had been at a party at the nearby Vauxhall Gardens Community Centre which was attended by approximately 100 other people.
Officers are appealing for any of the party guests who witnessed the attack to come forward.
DC Dominic Landragin from Lambeth CID said: "There were approximately 100 people attending the party at Vauxhall Community Centre. Enquiries show that there were several people outside immediately prior to, during and after the attack.
"Someone must have seen something. I would urge anyone with information to contact the police or Crimestoppers. This information may be key to bringing those responsible to justice."
Anyone with information is asked to call Lambeth CID on 101, tweet @MetCC or Crimestoppers anonymously on 0800 555 111 or online at www.crimestoppers-uk.org.