Kenya's Ruth Chepngetich won the women's marathon at the World Championships as 28 of the 68 starters withdrew in gruelling conditions in Doha.
In a race that started at midnight local time, Briton Charlotte Purdue was among the athletes to pull out in temperatures of 32C and with humidity reaching over 70%.
Organisers decided to go ahead with the event in its scheduled slot despite fears that the conditions might not be conducive for marathon running.
Linet Chebet of Uganda was taken away in an ambulance after failing to complete the distance, while Italy's Sara Dossena, another who did not finish, was seen in a wheelchair.
Ethiopia's marathon coach Haji Adillo Roba witnessed his trio of athletes stop, including Tokyo Marathon winner Ruti Aga.
"We never would have run a marathon in these conditions in our own country," he told BBC Sport.
Chepngetich said: "It was a tough race but I knew what to expect as I ran in Dubai.
"I trained for this weather running in the afternoon when the sun was high."
The 23-year-old took the first gold of the championships in two hours 32 minutes 43 seconds with athletes attempting to complete six 7km loops of the Corniche in the Qatar capital.
Bahrain's defending champion Ruth Chelimo, 39, was second in 2:33.46 and Commonwealth champion Helalia Johannes of Namibia, also 39, took bronze in 2:34.15.
Purdue, the third fastest British female marathon runner in history, withdrew after the start of the third loop. Compatriot Tish Jones pulled out before the race with a leg injury.
Chepngetich was among a small group that included Chelimo, two-time champion Edna Kiplagat, Kenya's Visiline Chepkesho and Johannes that broke clear from the rest as early as the fifth kilometre.
Israel's European champion Lonah Chemtai Salpeter was among the pack but dropped out as the quartet stretched away from her.
It became a two-horse race between Chepngetich and Chelimo before the eventual winner surged again, this time decisively, to secure her first major championships medal.
"I was not expecting to be a medallist in such tough conditions," Chelimo said.
'Acceptable level of health risk'
Earlier, athletics' governing body the IAAF had issued a statement confirming the marathon would go ahead, saying it had "done everything possible to minimise the heat-related risks" and that the race would be "run at an acceptable level of health risk".
But Belarus runner Volha Mazuronak, who came fifth, was angry with IAAF officials after the race: "The humidity kills you. There is nothing to breathe. I thought I wouldn't finish," she said.
"It's disrespect towards the athletes. A bunch of high-ranked officials gathered and decided that it would take the championships here but they are sitting in the cool and they are probably sleeping right now."
Canadian athlete Lyndsay Tessier, who came ninth, said: "You see somebody down on the course and it's just, extremely grounding and scary, that could be you in the next kilometre, the next 500 metres.
"I'm just really grateful to have finished standing up."
Most events at the championships are being held in the air-conditioned Khalifa Stadium, where the climate is maintained at 23-25 degrees.
However the marathon and Saturday's 50km walk are taking place outside the cooled venue. France's Yohann Diniz will be attempting to retain his world title in the 50km walk but is not happy about having to compete in Doha's heat and humidity.
"I am extremely upset," he said. "If we were in the stadium we would have normal conditions, but outside they have placed us in a furnace, which is just not possible.