Detectives are hunting up to nine gang members after a schoolboy was killed while "selflessly protecting" his younger brother.
Four suspects were 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.
Relatives and a host of school friends arrived to lay flowers as teachers and friends of the teenager came forward to pay tribute to him.
Noel, who declined to give his surname, said: "It was just a disgusting act of violence.
"I pray that a change happens, I pray for the guy that did this to my cousin."
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.
"He was a good guy, he was my cousin, someone I am very close to," Noel added.
"He was such a mature individual, so grown up.
"He was intelligent, he loved his family and I loved him."
Another cousin, Naomi, said: "We just hope his death is not in vain."
A picture of Negus was put up on an RIP page on Facebook, with friends paying tribute to him.
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.
Cousin Noel said he could not condemn the killer as "I am just another man".
He added: "I will let God decide. He needs help, he is just a young boy himself."
Negus "loved making music" and was destined for "something good".
"His mum was very proud of him," Noel added.
"His mum and dad are keeping it together.
"His brother and sister are keeping strong and needing their faith."
Negus's teachers at Edmonton County School spoke of their shock.
Headteacher Susan Tranter said he was a "polite young man".
"On behalf of the students, parents, staff and governors of Edmonton County School, I express my sorrow and deepest sympathies to Negus's family," said Dr Tranter.
"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."
A woman, who lives near the scene but did want to be named, said the victim was cradled by a crying woman as emergency services were called.
The stabbing took place near the doorstep of Michael Athnasiou's terraced home in Westminster Road, Edmonton.
He said there was now an atmosphere of fear in the area.
"The gangsters are getting out of control," he said.
"It is very scary."
Negus is the fourth teenager to be stabbed to death in the capital this year.
He died of multiple stab wounds after police were alerted to the attack at 7.25pm last night.
Mr Clayman confirmed the teenager had been stabbed in the chest and thigh.
He said: "We would urge anyone with any information about the murder or any witnesses to the incident in Westminster Road to come forward.2
:: Witnesses are asked to call the incident room on 020 8345 3775 or Crimestoppers on 0800 555 111.