A 15-year-old boy has been stabbed to death in a large fight after a birthday party attended by up to 100 youths in east London descended into violence.
Police were called to the North Romford Community Centre on Saturday night after fighting broke out as youngsters left the event.
The boy was found with knife wounds and died at the scene.
Scotland Yard said three male teenagers have been arrested on suspicion of murder and taken for questioning at separate police stations in east London.
The Met also deployed a large number of officers in the scene as a murder probe got under way.
Police were first called to the community centre on Clockhouse Lane at 9.05pm.
There had been 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."
The Met said a bus driver aged in his fifties was treated for a head injury at the scene, although there were no other injuries.
The boy's next of kin have been informed, although his identity has not been released.
Detectives have appealed for information over the incident.
DS Singh said: "I would like to directly appeal to those present to contact police about what they saw and heard to help my team build a clearer picture of what took place.
"Equally, if you have video footage or images of the violent aftermath to the party, we need to hear from you."
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."
After visiting the area, Andrew Rosindell, the MP for Romford, said it was "devastating" that a young life had "so tragically been cut short on our streets tonight".
Mr Rosindell tweeted: "I have visited Collier Row tonight to speak to Police at the scene of a very serious incident.
"A stabbing has taken place at the corner of Lynwood Drive and Clockhouse Lane, resulting in the death of a 14 year old boy, following a private party at North Romford Community Centre.
"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.