The party had been attended by up to 100 youths and the fighting broke out as the group left the event at North Romford Community Centre on Saturday night.
The 15-year-old was found with knife wounds and died at the scene.
His family has been told but his identity has not been released.
Police were called just after 9pm after reports of fighting and damage to the community centre in Clockhouse Lane.
Three youths have been arrested on suspicion of murder and are being questioned at separate police stations in east London.
A bus driver aged in his 50s was treated at the scene for a head injury but there were no other injuries reported.
Detective Sergeant Gurj 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: “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, local MP Andrew Rosindell said news of the boy’s death was “devastating” for residents.
“We must now wait for all the facts about this incident to be released,” he said.
“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.