Hooded gatecrashers are thought to have stormed into a party and stabbed a teenage boy, leaving him in hospital for two days.
Police are appealing for partygoers to get in touch after the incident at a house in St Benet's Grove, Carshalton, between 10.30pm and 11.30pm on Friday, December 14.
A number of youths in black hoods crashed the house party which had spilled out on to grass in front of the property. One of the group is described as much shorter than the rest.
The victim, a 16-year-old boy, was attacked with a knife and suffered puncture wounds and lacerations to his arm and shoulder. The boy was taken to hospital and stayed there for two days while he was treated.
Four youths were arrested on the scene.
Detective Constable Brian Gillespie wants anyone who was at the party or saw what happened to get in touch by calling Sutton CID on 020 8649 0747 or the charity Crimestoppers on 0800 555 111.