The Metropolitan Police said their had a been a leak of “noxious fumes”.
London Mayor Sadiq Khan said: “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 London Ambulance Service. Please avoid the area which has been cordoned off and evacuated.”
A mother in a swimsuit and her baby were seen being checked over by paramedics in a nearby cafe.
A swimmer who was in the pool at the time, but declined to give his name, said: “We were in the pool when the alarm sounded.
“I knew it wasn’t a drill because the staff were shouting to get out. A sign was also put up for people who couldn’t hear what was being said.
“We quickly got out and went through the changing rooms to run and grab our stuff and thankfully we could, because I don’t know how others are going to get their clothes.
“There was a really strong smell of chlorine in the changing room.
“I feel okay now, but the longer you were in there it got harder to breathe.
“We got out and it was okay. A paramedic came over to us and asked how we were doing, but we were fine – maybe a small headache.”
He added: “It was pretty scary. It’s a warm day so you feel like you should be able to go for a swim.”