By Jenni Marsh, May 23 (CNN)- Twenty-one ultra-marathon runners have died after extreme weather conditions hit a 100-kilometer (62-mile) mountain race in northwest China.

The high-altitude Huanghe Shilin Mountain Marathon began on Saturday morning in sunny conditions. But by 1 pm local time weather conditions had turned, with freezing rain, hailstones, and gale winds lashing runners in Gansu County, according to the state-run Global Times.

As temperatures dropped in the Yellow River Stone Forest, runners started to report suffering from hypothermia, while others went missing.

The marathon organizers called off the race and launched a search party of 1,200 people to scour the complicated terrain. The search operation continued after dark.

Most competitors were wearing thin shorts and T-shirts.

One participant told local publication Red Star News: "At one point, I couldn't feel my fingers (because it was so cold). At the same time, my tongue felt frozen, too."

He said he decided to abandon the race. "I retreated back to halfway down the mountain and entered a wooden cabin in the direction of a rescuer. There were already about 10 more runners who came down earlier and we waited for rescue in the cabin for about an hour.

Eventually, about 50 runners came and took shelter in the cabin."

By Sunday morning, 151 of the 172 race participants had been confirmed safe, with eight in hospital. Another 21 were found dead, according to the state-run People's Daily.

The race's distance of 100 kilometers was more than double that of a standard marathon.