The Manitoba Marathon has been cancelled after Environment Canada issued a heat warning, organizers confirmed in a Facebook message to CBC News.

About an hour after the run started, the decision was made to cancel the event, which boasted about 6,000 participants.

They crossed the start line at Winnipeg’s IG Field at 7 a.m. as the temperature was already a balmy 23 C, but the humidex values will likely climb into the 35 to 40 range by 10 a.m.

Earlier in the race, Manitoba Marathon executive director Rachel Munday said organizers were prepared for the heat, but they would be ready to close parts of the course