At least 12 people suffered burns after a man sprayed acid inside a club in east London.
Hundreds of revellers were evacuated from Mangle E8 in Sidworth Street, Hackney, after police were called at about 01:10 BST.
Officers believe a man threw acid at two men after a dispute between two groups inside the venue.
Two men in their 20s remain in a serious but stable condition in hospital.
Ten other people were were treated for minor injuries, the Metropolitan Police said.
A London Fire Brigade spokesperson said: "The only information that we have is that it was an unknown corrosive substance thrown within the nightclub.
"It was identified by a pH paper test as a strong acidic substance."
It is understood about 600 people were at an event in the club.
London Ambulance Service said it sent a number of ambulance crews and a hazardous area response team to the venue.
Officers said 10 people were taken to hospital by ambulance, while a further two sought treatment at A&E.
They said no arrests had been made and officers were continuing to investigate.
The police added there was no evidence to suggest the attack was gang related.
Witness Phie McKenzie tweeted: "Scary scenes tonight, we have heard reports of chemical burns of people in the building here".
A woman who lives in a block of flats overlooking the nightclub described a scene of confusion as crowds were moved up the street away from the venue.
The tenant, who asked not to be named, said: "It happened so fast - they rushed people out. I thought it was a stabbing because all I heard was 'move, move, move - move out of the area'."
She added: "The security were telling the crowd to go down that way - people on this side were waiting to go home but they couldn't go home because the road was closed. People were confused."
A LoveJuice club night was being held at the venue and the company behind the event tweeted to say it was cooperating with the police investigation.