An official wraps an exhausted athlete in a foil blanket at the finish line of the Vancouver sprint distance triathalon on Friday morning. ((CBC))

Some of the world's top athletes had to seek medical attention after competing in what may have been the coldest swim in World Championship Triathlon history in downtown Vancouver.

The preliminary sprint distance race kicked off early Friday morning in unseasonably cool weather when about 600 competitors — lashed by wind and rain — hit the 11-degree water for a 750-metre swim in English Bay.

Then, with the air temperature hovering around eight degrees, the athletes jumped on their bikes for a 20-kilometre ride followed by a five-kilometre run through Stanley Park wearing only light Lycra triathlon suits.

That's when things really got chilly, according to Jeff Hayne, one of the dozens of athletes shivering under a reflective Mylar blanket in the medical tent after the race.

""It was about as cold as I have done in Canada... even Fort McMurray 12 years ago wasn't this cold. To get out and get on a fast downhill is what really got you in a bad spot. [During] the run, you finally warm up a little bit," said Hayne.

Dr. Sam Gutman said it was controlled mayhem in the medical tents at the finish line where many athletes collapsed from hypothermia.

Ambulances had to be used to warm up the athletes because the warming tents were too cold. It got close to the point where it was not safe to continue the event, said Gutman.

"The muscles don't work properly... [Athletes] get very cold. They get cramps. They get nauseated. They get confused…their level of consciousness will change, so they tend to push themselves harder. So, they can get themselves into trouble very quickly," he said.

The real test will be Sunday when three times the number of athletes will participate in the main event, said Gutman, and if temperatures dip any lower, organizers may have to reconsider plans for the event, he said.

The forecast for Sunday is a mix of sun and cloud with 40-per-cent chance of showers, a low of 10 degrees and high of 16.