The London Ambulance Service has called it a "major incident" and said several people were being treated at the scene.
Several people suffered "breathing difficulties" at the London Aquatics Centre following the release of gas, the Queen Elizabeth Olympic Park said in a statement on Wednesday.
"A high quantity of chlorine gas was released" due to a chemical reaction at the Aquatics Centre, London's Fire Brigade specified in a statement.
Around 200 people were evacuated from the centre, which is now being ventilated. The London Ambulance Service confirmed they were treating a number of patients.
"While we ventilate the premises we would ask those residents in the immediate vicinity to close doors and windows," the London Fire Brigade spokesperson said.
The ambulance service called it a "major incident" at the centre which held aquatics events during the 2012 Olympics.
Images posted to social media showed multiple ambulances and fire engines at the scene.
London Mayor Sadiq Khan said he was in close contact with emergency services and said a number of people were being treated.
"Please avoid the area which has been cordoned off and evacuated," Khan said in a tweet.