A house party descended into violence after as many as 300 people turned up, Scotland Yard said.
Two teenage boys were arrested and three large knives confiscated after disorder broke out at the event in Dagenham, east London, last night.
The Metropolitan Police said officers were called to reports of a "large fight" in Oxlow Lane just after 11pm, arresting a 15-year-old boy for assaulting a police officer and a 17-year-old for possession of an offensive weapon. Both are being questioned at an east London police station this morning.
Inspector Jason O'Donohue said: "Three large knives were recovered by my officers as they worked to disperse a very large and unpredictable crowd following a private house party. By getting these knives off the street when they did, my officers have helped save lives and prevent another needless tragedy."
Police said the crowd of "between 200-300 house party visitors" was dispersed, with no reported injuries.