Gunmen in Halloween masks opened fire with automatic weapons on revellers at an illegal rave.
At least three men unleashed a hail of bullets inside and outside the makeshift club on an industrial estate.
One clubber appears to return fire, with a handgun, video from the event shows.
A business owner, whose property was hit by a stray bullet, said: “It looked like it was a gangland execution gone wrong.”
The shooting happened at the Money Move rave in Leyton, North East London.
One partygoer said: “There must have been at least four guns. It was terrifying. Everyone who had got out was just running but there were over 100 trapped inside. I turned and ran.
Video from the scene shows the wounded man being helped by a woman using a belt as a tourniquet.
The venue, described as an African church, is said to be often hired for raves.
The Met Police would not speculate on the motive for the shooting. A spokesman added: “There have been no arrests. Enquiries continue.”