Police investigating the knife murder of an 18-year-old male youth in south London said on Friday he had been assaulted by up to eight hooded attackers.
The victim, who died on Thursday evening, is the 21st teenager to be killed violently in the capital this year.
Officers arrested a 16-year-old boy over the murder at his home in Brixton in the early hours of the morning.
Police had been called on Thursday evening to reports of a fight in Guildford Road, south Lambeth, and found the 18-year-old stabbed in the abdomen.
He was taken to a nearby hospital where he was pronounced dead.
"It is believed that up to eight males were involved in the assault on the victim," Scotland Yard said.
"The suspects are described at this early stage as wearing hooded tops and some were possibly in possession of bikes."
Police say they are aware of the victim's identity and have contacted his next of kin but are waiting for a formal identification before releasing further details.
Knife crime has become a top political issue in recent weeks with a spate of violent deaths, mostly stabbings, on the streets of London so far this year.
On Friday the funeral took place of 16-year-old Ben Kinsella, who was stabbed to death on June 29 after celebrating the end of school exams at an Islington bar in north London.
Official figures released on Thursday revealed there were more than 22,000 serious offences in England and Wales involving knives in 2007-8, although crime fell significantly overall.
