London Ambulance Service and London Fire and Rescue were both called to the Aquatics Centre on Wednesday morning with casualties being treated for breathing difficulties.
The area has been evacuated and cordoned off, and people have been urged to stay away.
In a tweet, LAS said: “London Ambulance Service is responding to a major incident @AquaticsCentre in Stratford alongside colleagues from @LondonFire and @metpoliceuk.
“We have dispatched multiple resources to the scene and are treating a number of patients.”
The Olympic Park Twitter account added: “There has been an incident @AquaticsCentre this morning involving the release of a gas. The area has been cordoned off and evacuated.
“We’re working with emergency services on site. There are a number of casualties with breathing difficulties being treated by @Ldn_Ambulance.”
Mayor of London Sadiq Khan added: “I remain in close contact with our emergency services who are dealing with a gas-related incident at the London Aquatics Centre this morning.
“A number of people are being treated by London Ambulance Service. Please avoid the area which has been cordoned off and evacuated.”