The London Ambulance service is responding to an incident at Queen Elizabeth Olympic Park.
The incident reportedly took place at the aquatics centre in the morning and involved a gas leak.
Several people with respiratory problems are being treated at the scene, Reuters quoted the park's spokesman as saying.
The area has been cordoned off and evacuated, the spokesman added.
"Due to a chemical reaction a high quantity of chlorine gas was released. London Ambulance Service are treating a number of patients', a London Fire Brigade spokesman said, quoted by the Daily Mail.
He added that some 200 people were evacuated from the centre. Some of them were suffering from breathing problems and headaches.
The London Ambulance Service were called the the Olympic Park in east London at just before 10am after a noxious substance leaked into the building containing the swimming pool and diving area.
People described suffering headaches and struggling to breathe because of the fumes. Some were seen lying on their backs on the pavement.
Mayor of London Sadiq Khan asked Londoners to avoid the area.
"I remain in close contact with our emergency services who are dealing with a gas-related incident at the London Aquatics Centre this morning...Please avoid the area which has been cordoned off and evacuated," the mayor tweeted.
Photos from the park were shared online, showing ambulances and medics working at the scene.
Ambulance crews are responding to reports of a gas leak at the Queen Elizabeth Olympic Park Aquatics Centre in east London
© Photo : 3li/twitter
The London Ambulance service said it will keep the public updated on the incident.