Two people were seriously injured and ten more suffered burns after a man sprayed acid at nightclubbers in east London.
Emergency services, including a hazardous area response team, were sent to the Mangle club in Dalston in the early hours of Monday morning, closing surrounding roads.
Detectives said they understand a dispute between two groups of people developed inside the venue. The suspect remains at large.
As the row escalated, it is believed a noxious substance was sprayed by a male suspect directly at two men.
Both are in their twenties and described as being in a critical but stable condition in hospital.
Other people inside the venue, which can hold as many as 600 revellers, suffered the effects of the substance.
The substance is yet to be identified, but it is believed to have been acidic, police said.
The Metropolitan Police added: “A police investigation is underway. There has been no arrest at this early stage and enquiries continue. There is nothing to suggest that this is gang related.
“Anyone with information that may assist police should call detectives from Hackney CID via 101.
“To remain anonymous, call Crimestoppers on 0800 555 111.”