LONDON (Reuters) -A number of casualties with breathing difficulties were treated by London's ambulance service on Wednesday after a leak of a high quantity of chlorine gas at the Queen Elizabeth Olympic Park's aquatics centre, emergency services said.

       The London Fire Brigade said that around 200 people had been evacuated after it was called at around 0945 GMT to the east London park, which hosted Summer Olympics events ten years ago.

       "Due to a chemical reaction a high quantity of chlorine gas was released. London Ambulance Service are treating a number of patients," London Fire Brigade said in a statement.

       "While we ventilate the premises we would ask those residents in the immediate vicinity to close doors and windows."

       The London Legacy Development Corporation (LLDC), which manages the park, said the immediate area had been cordoned off.

       The London Ambulance Service described it as a major incident, and said it had sent multiple resources to the scene. Pictures on Twitter showed more than a dozen ambulances outside the swimming pool.

       (Reporting by Muvija M and Alistair Smout, Editing by Kylie MacLellan)