A London Fire Brigade spokesman told AFP an “unknown corrosive substance” was thrown in Mangle, a club in east London, in the early hours of Monday.
It was identified as “an acidic substance” from testing, he said, adding that there were around 600 people in the venue at the time of the incident.
“Twelve people showing signs and symptoms attributed to corrosive substances were treated on scene by ambulance and brigade personnel prior to removal to hospital,” he added.
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.
In 2016, corrosive fluids were used in 454 crimes, compared to 261 in 2015.
— Phie McKenzie (@PhieMcKenzie) April 17, 2017
Some experts have suggested that criminal gangs may be switching from carrying knives to acid because it could lower the risk of prosecution.
In an acid attack in north London earlier this month, a 40-year-old father, 36-year-old mother and their three-year-old son were hurt.
The woman and child were later discharged from hospital but the man suffered “life-changing injuries”, police said.