The incident in Dalston, east London, forced the evacuation of the venue where around 600 revellers had been at an event, it is understood.
Roads were closed as emergency services, including a hazardous area response team, rushed to the scene after the alarm was raised in the early hours of Monday morning.
The London Ambulance Service (LAS) said it had taken 10 patients to hospital, while police said a further two people with similar injuries sought treatment at a hospital.
A witness tweeted: “Scary scenes in Hackney tonight, we have heard reports of chemical burns of people in the building here.”
A Scotland Yard spokesman said: “Police were called to a licensed premises in Sidworth Street, Dalston, at approximately 1.10am on 17 April, after members of the public complained of a noxious substance.
“London Fire Brigade and London Ambulance Service were called to the scene.
“Ten people have been treated for minor burns at the scene and taken to east London hospitals by LAS.
“Another two people presented themselves at an east London hospital with similar injuries.
“All victims’ injuries are non-life threatening.”
No arrests have been made and police are investigating the incident.
Stuart Crichton, LAS assistant director of operations, said: “We were called at 1:09am today, Monday 17 April, to reports of an incident on Warburton Road, E8.
“We sent multiple resources to the scene including paramedics in cars, ambulance crews, a number of officers an advanced paramedic and our hazardous area response team.
“We have taken 10 patients to hospital.”
Around 200 people had left the premises before the arrival of emergency services, with a further 400 evacuated from the building.