Firefighters dealt with a chemical incident at the Aquatics Centre on Queen Elizabeth Olympic Park in Stratford.
London Fire Brigade spokesperson said:
“Due to a chemical reaction a high quantity of chlorine gas was released. London Ambulance Service treated a number of patients. Around 200 people were evacuated from the centre.
"Fire crews used fans to ventilate the premises. Once low levels were reported on our detection equipment we handed the centre back to the building owners."
The Brigade was called at 0945 and the incident was over by 1529. Fire crews from Stratford and surrounding fire stations were at the scene.