Police were called to Oxlow Lane, Dagenham, at 23:00 BST on Saturday after reports of a street fight.
Officers recovered three large knives, scissors and cannabis from the "large and unpredictable" crowd, they said.
Two boys aged 17 and 15 were arrested - one on suspicion of possessing an offensive weapon and the other on suspicion of an assault on police.
Scotland Yard said no one had been injured at the event.
Insp Jason O'Donohue said: "By getting these knives off the street when they did, my officers have helped save lives and prevented another needless tragedy."
Two teenagers have been arrested and three knives recovered after 300 people attended a house party in east London.