Staff at The O2 have been taken to hospital after inhaling ammonia.
A spokeswoman for the London Fire Brigade said 30 workers at the venue in Greenwich, formerly the Millennium Dome, were treated for minor injuries after the gas leaked from the ice rink plant to the outside of the arena through an air vent.
No members of the public were harmed and the London Fire Brigade and Health and Safety Executive have launched investigations.
The fire brigade was called at 8.30pm on Monday, with police and the London Ambulance Service also attending.
The LFB spokeswoman said: "The gas supply was isolated and monitoring of the affected area was carried out using specialist equipment.
"The area has now been declared safe. An investigation team has been on site but the matter is now in the hands of the site manager."
The LFB spokeswoman said a total of 30 members of staff suffered ammonia inhalation and 27 were taken to hospital, with three declining treatment. The staff complained of runny noses, itchy eyes and discomfort.
A spokeswoman for The O2 said: "There was a minor ammonia leak last night at The O2. The building was immediately evacuated. Some members of staff have gone to hospital as a precautionary measure."
The O2 opened in June this year in what was formerly known as the Millennium Dome. The entertainment venue's centrepiece is a 20,000-capacity arena. It has already attracted some of the biggest names in music to perform, including Prince, Bon Jovi and Sir Elton John.
In November an ice disco is due to open for four months on a 900-square-metre ice rink in the London Piazza area of the venue.
