A 16-year-old boy was stabbed to death in an east London park after leaving a friend's birthday party.
The teenager had been celebrating a girl's 16th birthday on Saturday with around 100 other young people at a social club on the Ashton playing fields in Woodford.
The boy, who was alone, got into an argument with a gang of four male youths outside the building before one stabbed him. The youths then fled towards Chigwell Road.
Other people at the party tried to save the victim before police and paramedics arrived at around 9.40pm, but he died just over an hour later at Whipps Cross University Hospital.
The boy has not been named or formally identified, though police believe they know his identity. Next of kin have been informed and a post-mortem examination will take place on Monday.
No arrests have been made and the killing is being investigated by detectives from the Homicide and Major Crime Command.
DCI Ken Hughes, leading the investigation, said: "There were around 100 young people at this party and we are appealing for anyone with information or any witnesses to come forward as soon as possible.
"The victim's family are naturally devastated and in shock by what has happened and we need the help of those at the party that night to progress our investigation."
It is the first murder of a teenager in the capital this year, Scotland Yard said.
Nineteen teenagers were murdered in London in 2015, of whom 15 were stabbed to death. Two others were strangled, one suffered head injuries and one died from "other" causes.
Anyone with information is asked to call police on 020 8345 3862 or Crimestoppers on 0800 555 111.
The boy was named locally as Charlie Kutuyampo, though Scotland Yard could not confirm his identity.
Police this afternoon recovered two knives from near the scene, each around 5in long, and they were taken away by forensics officers.