A number of people are being treated for breathing difficulties after a “high quantity of chlorine gas” leaked in Queen Elizabeth Olympic Park in east London.
Around 200 people were evacuated from the Aquatics Centre on Wednesday morning, firefighters from London Fire and Rescue told reporters at the scene.
People could be seen being placed into ambulances while others were wrapped in blankets.
Residents in the area have been asked to close windows and doors.
Dozens of emergency service vehicles and workers were in the area, with surrounding roads cordoned off and members of the public denied access to the park.
Firefighters said the plan is to ventilate the centre to disperse the chlorine into the atmosphere.
The brigade tweeted: “Update Aquatic Centre in #Stratford. Due to a chemical reaction a high quantity of chlorine gas was released inside the centre.
“While we ventilate the premises we would ask those residents in the immediate vicinity to close their doors and windows.”
In a statement, the Metropolitan Police said: “Officers were alerted by London Fire Brigade at 9.53am on Wednesday to a leak of noxious fumes at the Aquatics Centre, Queen Elizabeth Park, E20.
“Officers and London Ambulance Service have also attended the scene.
“A number of people have been evacuated from the premises.”
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 said: “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.”
A London Legacy Development Corporation spokesperson said it is working to ensure the park can reopen on Thursday.