Emergency services are responding to a “major incident” at the London Aquatics Centre in Stratford and are treating “a number of patients”.
The London Ambulance Service confirmed they were working alongside the London Fire Brigade and the Metropolitan Police at the scene.
Videos circulating on social media showed several ambulances rushing to the scene.
The London Fire Brigade said they were dealing with reports of “a chemical incident” at the centre in Queen Elizabeth Olympic Park.
In a tweet, Queen Elizabeth Olympic Park wrote: “There has been an incident at @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.”
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Mayor of London Sadiq Khan tweeted: “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 @Ldn_Ambulance.
“Please avoid the area which has been cordoned off and evacuated.”