Almost 30 people have been taken to hospital with 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, in what was called a "major incident."
Ambulance crews took 29 people to hospital and assessed a further 48 at the scene, the majority experiencing minor breathing difficulties.
People could be seen being placed into ambulances while others were wrapped in blankets.
Residents in the area were asked to close windows and doors.
Dozens of emergency services vehicles and workers were in the area, with surrounding roads cordoned off and members of the public denied access to the park.
Firefighters began to ventilate the centre to disperse the chlorine into the atmosphere by 3pm (1500 GMT).
Darren Farmer, London Ambulance Service gold commander, said: "We sent a significant number of resources including 13 ambulance crews, advanced paramedics, two medics in fast response cars, two incident response officers, a medical incident adviser and members of our hazardous area response team (HART)."
