A 16-year-old boy has been found stabbed to death in a stairwell in a block of flats after a party.
The boy was pronounced dead after being found in the ground floor of the block in Wandsworth, south west London, at 1.30am on Sunday, said Scotland Yard.
An undisclosed number of people have been arrested in connection with the murder, said a Metropolitan Police spokesman.
Detective Chief Inspector Nick Scola, of the Metropolitan Police's homicide and serious crime command, said police wanted to speak to a group of between five and eight men seen running from the scene at Albon House in Neville Gill Close.
He said: "I believe the victim had attended a party in Albon House. I need to trace everyone at that party, and in particular I am appealing for any information about a group of between five and eight males seen running from the scene into the surrounding streets and possibly through King George's Park."
The next of kin of the boy, who has not yet been formally identified, have been informed, but officers await formal identification, he said.
The people arrested already are in custody at police stations in south and central London.
Police cordoned off a stretch of road outside Albon House as forensics teams examined the scene.
Jessica Cremin, 19, who lives nearby, said: "I have a lot of younger cousins and brothers and sisters that I take out to the park, so obviously now I'm going to be more aware of what's going on.
"It's really frightening. When I heard this had happened I had shivers down all over my body and I was shaking. It's so scary."