A 'major incident' has been declared at London's Olympic Park after noxious fumes left a number of people unable to breathe.
Emergency vehicles descended on the Aquatics Centre at the park in Stratford, East London after a large amount of chlorine was released.
Around 200 people were evacuated from the centre - including groups of school children, according to witnesses.
London Ambulance Service said 29 people were taken to hospital and a further 48 people were treated at the scene - with most patients reporting breathing difficulties.
Some were reportedly treated for breathing difficulties in ambulances waiting nearby.
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London Fire Brigade spokesperson said: “Due to a chemical reaction a high quantity of chlorine gas was released. London Ambulance Service are treating a number of patients.
"Around 200 people were evacuated from the centre. While we ventilate the premises we would ask those residents in the immediate vicinity to close doors and windows.”
A spokesman for the Aquatics Centre added the release of gas came as it took delivery of a number of chemicals.
They added on Twitter: "At approximately 10am this morning we took the decision to evacuate the London Aquatics Centre.
"This was due to a release of gas that occurred when the facilities management company that operates the plant room took delivery of pool chemicals.
"We are awaiting guidance from the facilities management company, LLDC and the emergency services on when the Centre will be able to reopen."
London Ambulance Service Gold Commander Darren Farmer said: “This morning we responded to a major incident at the London Aquatic Centre in Stratford, alongside colleagues from the London Fire Brigade and Metropolitan Police.
“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 advisor and members of our Hazardous Area Response Team (HART).
“We also dispatched medics from London’s Air Ambulance.
“Our crews took 29 patients to hospital and assessed a further 48 patients at the scene. The majority of patients were reporting minor breathing difficulties.
“Thank you to all our teams who responded today, both at the scene and in our control room, and to our firefighter and police colleagues.”
London Fire Brigade said: "Firefighters are dealing with reports of a chemical incident at Aquatics Centre on Queen Elizabeth Olympic Park. @Ldn_Ambulance are treating a number of patients."
The official Twitter for the Queen Elizabeth Olympic Park said: "There has been an incident @AquaticsCentre this morning involving the release of a gas.
"The area has been cordoned off and evacuated. We're working with emergency services on site. There are a number of casualties with breathing difficulties being treated by @Ldn_Ambulance ".
Mayor of London Sadiq Khan urged Londoners to avoid the area while he liaised with emergency services.
He wrote on Twitter: "I remain in close contact with our emergency services who are dealing with a gas-related incident at the London Aquatics Centre this morning.
"A number of people are being treated by @Ldn_Ambulance. Please avoid the area which has been cordoned off and evacuated."
Images from the scene show huge crowds of emergency personnel, workers and those who were using the swimming pool.
Some people were reportedly ordered to evacuate immediately while still in their swimwear.
Mirror reporter Ashleigh Rainbird said groups of school children appeared to be at the pool at the time and were being safely evacuated from the building.
She said a huge cordon remains in place, adding: "There's quite a lot of construction work going on outside and they all had to down tools as well.
"When I was past I saw groups being evacuated. When I went back to look, I heard someone say some children were being given oxygen.
"They have taken people to a nearby pub where some are sitting in blankets.
"The cordon is very big and it is very close to Westfield shopping centre, which is still open."
The Metropolitan Police said: "Officers were alerted by London Fire Brigade at 9.53am on Wednesday, March 23 to a leak of noxious fumes a the Aquatics Centre, Queen Elizabeth Park, E20.
"Officers and London Ambulance Service have also attended the scene. A number of people have been evacuated from the premises."
The swimming pool is located within the famous Olympic Park, home to the 2012 summer Olympics.
It is also located near the huge Westfield Stratford shopping centre, one of the busiest indoor shopping centres in the country
