Zhang Xuchen, mayor of Baiyin, Gansu province, where the tragedy occurred, bowed in apology to victims’ families on Sunday.

“This is a public safety incident caused by sudden change of regional weather. As the event organiser, we are in deep regret and remorse. We apologise to the families of the victims and injured [athletes] and our condolences go to you. The provincial government has also set up an investigation task force to follow up the investigation,” Zhang said.

In a commentary, state broadcaster CCTV said extreme sports should be properly organised to avoid such a rare and catastrophic tragedy.

Marathon and ultra long distance running has gained popularity in China in recent years as the country’s growing middle class have taken up the pastime.

Trail races organised in remote areas offer lavish prize money to attract participants. Regional governments in western China including Yunnan, Gansu and Sichuan, set aside funds to host sporting competitions as part of the official tourism development strategy.

The runners in Saturday’s event set off in the morning from the Yellow River Stone Forest Park in Baiyin. Bad weather set in at about 1pm as they climbed to a height of about 2,000 metres (6,500 feet) above sea level during the 20km-31km section of the race, state news agency Xinhua quoted organisers as saying.

Dressed in only shorts and tops, they had little protection against the freezing temperatures. When it became clear some of the runners were in trouble, the event was cancelled and a rescue effort was launched.

By that time, several people were suffering from hypothermia and others had passed out due to the cold.

In all, 21 were confirmed dead, and the remaining 151 competitors were rescued, of whom eight sustained minor injuries.

02:46

21 runners die in freezing weather during ultramarathon in northwestern China

21 runners die in freezing weather during ultramarathon in northwestern China

One survivor, who declined to be named but whose identity was confirmed by the Post, said in an online account that there was no forecast of the sudden extreme weather and the cold worsened the challenges of the terrain.

He said he continued even after the bad weather set in but saw other runners ahead of him, including elite athletes, drop out one by one.

Race organisers offered each runner 1,600 yuan (US$248) to complete the event and at the time he thought to himself “is this how you give up the 1,600 yuan?”

“Now every time I recall that thought, I just want to slap myself,” he wrote in a widely shared social media post.

He said that in one section, the competitors had to climb 1,000 metres in an 8km stretch over rocky, muddy ground in strong winds, freezing rain and low visibility.

“From this point onwards, there was only one way up and no way down ... It’s a section even motorcycles can’t reach so there was no supplies including food and water even after reaching the mountain top,” the runner wrote.

“I was completely drenched and could barely stand. I found a spot where I tried to warm up with a thermo blanket but it was blown apart by the wind. I saw other runners’ blankets were shredded into pieces by the strong wind.”

The competitor said he soon could not feel his fingers.

“I put my fingers into my mouth trying to warm them up but I still felt nothing after ... even my tongue felt frozen,” he wrote.

“That’s when I decided to drop out and descend. Lucky I made that decision as I was already showing symptoms of hypothermia.”

The runner said he saw many of the competitors passed out on the way down but he and others like him did not have enough in them to help others.

He also wrote that it was difficult to track down some runners with their GPS signals due to poor communication. Apart from hypothermia, some runners also sustained cuts and falls, he wrote.

Ma Shuzhi, one of the women who survived the event, was quoted by Chengdu news site Chengdu.cn as saying that she felt lucky to make it out alive.

“The biggest takeaway from the event is: to survive,” she was quoted as saying.

Rescue efforts were hampered by the altitude and bad weather, Xinhua reported, adding the park had been closed to visitors.

The mountain is an annual event that has been organised for the past four years by authorities in Jingtai county and Baiyin as part of a rural revitalisation drive and to promote healthy lifestyles, according to China National Radio.

Almost 10,000 people signed up for this year’s event, which was divided into three categories: beginners, 21km cross-country and 100km cross-country.

Additional reporting by Mark Agnew