A 15-year-old boy was stabbed to death in a large fight after a birthday party attended by up to 100 youths in east London.
Friends screamed "he's dead" and "they've killed my best friend" as groups of teenagers "scattered" following the attack, witnesses said.
The violence broke out at a community centre on Clockhouse Lane, Romford, east London, on Saturday night after scores of youths were apparently turned away with the venue at capacity.
Three male teenagers have been arrested on suspicion of murder and taken for questioning at separate police stations in east London, the Metropolitan Police said.
A witness who gave her name only as Carol said "it was chaos out here last night" and described hearing friends of the victim screaming and sobbing.
She told the Press Association: "Around 9.20pm I heard a lot of noise going on. For some reason it didn't sound right. It was mostly black boys out there.
"Then I heard a lad crying down the phone 'he's dead, he's dead'.
"He was lying down in the road, there was a lot of blood. The police were working on him, they tried their absolute damnedest with that boy last night."
A teenager, who saw the aftermath of the attack and gave her name only as Ellie, added: "There was a big group on the corner with just the boy on the floor.
"There were paramedics doing shocks and CPR on him for quite a while. The best part of an hour. After, he was just laying there with a white sheet over him, it was covered in blood."
A cordon remained in place on Sunday morning as investigators and forensic teams searched for evidence.
Local MP Andrew Rosindell said it was "devastating" a young life had "so tragically been cut short on our streets tonight".
A bus driver in his 50s was treated for a head injury at the scene, although there were no other injuries, Scotland Yard said.
Police were first called to the centre at 9.05pm after reports of fighting between groups and the centre being damaged.
Detective Sergeant Gurj Singh said: "Response officers who attended the initial calls were confronted with up to 100 youths leaving a birthday party gathering at a community centre.
"There appeared to be fighting and tragically a teenager has been found with fatal injuries."
Conor Rufus, 20, estimated there were actually up to 200 people on the street.
He said: "Some people were running toward the body, some people were running away, there were cars coming and picking people up.
"It was all kicking off. There was all this screaming."
The boy's family has been informed, although his identity has not been released.
The Metropolitan Police is investigating more than 75 homicides since the start of the year.
Response Team Inspector Michael O'Donoghue, from the Met's East Area, added: "Officers have been out in force this evening providing reassurance to those in the community who have no doubt been shocked by this appalling attack on a teenager.
"We will continue to provide a high-visibility presence and officers will be on hand to hear from those from the local community."
Mr Rosindell tweeted: "This is devastating news for the residents of North Romford. We must now wait for all the facts about this incident to be released.
"Until then, our condolences must go to the family of the young boy whose life has so tragically been cut short on our streets tonight."
Any witnesses or people with information are urged to call the incident room on 0208 345 3775 or Crimestoppers anonymously on 0800 555 111.
They can also give information online via crimestoppers-uk.org/give-information/forms/give-information-anonymously or tweet police via @MetCC.