At least 11 people have suffered burns from a suspected noxious substance at a bar.
Emergency services including a hazardous area response team were sent to the venue in Dalston, east London, in the early hours of Monday morning, closing surrounding roads.
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 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.
"Nine 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."