Twelve people have been reportedly injured after “an acidic substance” was thrown in Mangle, a club in east London, on Monday morning.
There were around 600 people in the venue at the time of the incident.
A London Fire Brigade spokesman said: “Twelve people showing signs and symptoms attributed to corrosive substances were treated on scene by ambulance and brigade personnel prior to removal to hospital.”
The police said in a statement that all the injuries were “non-life threatening” and the incident was not believed to be terrorism-related.
Police were called to the scene at around 0010 GMT “after members of the public complained of a noxious substance,” the statement said.
The streets around the club have been closed to traffic as the investigation continues and no arrests have been made.
London has seen a sharp rise in acid attacks in London in recent years.
There were more than 1,800 reports of attacks involving corrosive fluids in London since 2010, according to police data cited by the BBC last month.
A Scotland Yard spokesman said: “Detectives are investigating an incident in which acid was sprayed inside licensed premises in Hackney.
“Police were called to the premises in Sidworth Street, Dalston, at approximately 1:10am on Monday after members of the public complained of a noxious substance.
“London Fire Brigade and London Ambulance Service were also called to the scene.
“A total of 12 people attended hospital suffering burns. Ten people were taken by ambulance and two people presented themselves at an east London hospital.
“Two of those injured; both men aged in their 20s – remain in a serious but stable condition. The others were treated for minor injuries.
“Around 200 people had left the premises before the arrival of emergency services, with a further 400 evacuated from the building.”