A swimming pool in London’s Olympic park was evacuated Wednesday and several people were treated by ambulance workers after an incident involving the “release of a gas.”

The Queen Elizabeth Olympic Park in east London, built for the 2012 Games in London, said on Twitter that the incident took place at the Aquatics Center. It said “there are a number of casualties with breathing difficulties” who are being treated by ambulance workers.

It said the area was cordoned off and evacuated and emergency services were on site. The park didn’t provide more details.

The London Fire Brigade said firefighters were dealing with “reports of a chemical incident” at the pool.

The capital’s ambulance service said it was responding to a “major incident” alongside firefighters and police. It said its workers were treating a number of patients.

London’s mayor, Sadiq Khan, said he was in close contact with emergency services and warned people to avoid the park.

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The Associated Press contributed to this article.