Two teenagers were stabbed and two others were injured during an incident at a house party in Harrow.
Two boys, both 16, were stabbed and were taken to hospital where their injuries were assessed to be not life threatening.
Two other boys, aged 16 and 15, were treated for minor injuries to their hand and fingers.
No arrests have been made and a police investigation is continuing.
A Met statement said: “We’re investigating an incident at a house party in Tudor Road, Harrow at around 10.45 [pm] last night.”
The statement added: "The police helicopter was deployed to the area to support officers carrying out searches. There has been no arrest at this early stage.”
Any witnesses who are yet to speak to the police should call 101, giving the reference 7164/14JAN. Information can also be provided to Crimestoppers, anonymously, on 0800 555 111.