Detectives are hunting up to nine gang members after a schoolboy was killed while "selflessly protecting" his younger brother.
Four suspects have been arrested over the death of Negus McClean as relatives paid tearful tributes to the 15-year-old aspiring musician.
Family members hit out at the "disgusting act of violence" which led to his death but vowed to pray for his killers.
The 15-year-old's cousin, Noel, called for an end to knife crime. Fighting back tears after laying flowers at the scene of the attack in Edmonton, north London, the 25-year-old described Negus as "intelligent, mature and selfless".
"He was dying protecting his brother," he said. "It was a selfless act. He paid with his life."
Speaking at the murder scene, Detective Chief Inspector Stephen Clayman said Negus died after being chased by "seven, eight or nine" youths riding bikes.
Negus was preparing to celebrate his 16th birthday in two weeks' time and was hoping to embark on a career as a rapper after leaving school, relatives said.
The teenager's younger brother - believed to have recently started secondary school - saw Negus collapse after being attacked by four youths on bicycles after a fight over a BlackBerry phone, locals claimed.
Negus's teachers at Edmonton County School spoke of their shock. Headteacher Susan Tranter said he was a "polite young man".
"It is a tragedy that the life of a 15-year-old boy has been lost to knife crime. I urge all the young people of our school to shun the gang-related issues that will have contributed to this untimely death; there is no future for a young person as a member of a gang. Let us cherish the memory of Negus and make sure it never happens again," she said.
