OSAKA, Japan (AFP) - Soaring heat and humidity threaten to overshadow the World Athletics Championships with a number of casualties in the opening events.

Twenty-eight runners failed to finish the men's marathon -- the slowest in championships history -- with athletes collapsing around the course.

With 33 C heat and 78 percent humidity on the opening morning, it was no wonder the stretcher-bearers were kept busy.

On day two Jefferson Perez dropped to the track with uncontrollable spasms after winning the 20 kilometres walk in graphic testimony to the gruelling conditions.

Russian contender Valeriy Borchin needed medical attention after his legs gave way in the closing stages.

"This could be the toughest of my victories," Perez said. "I was walking so fast that in these conditions, I knew I could collapse.

"It was kind of suicidal. Luckily I went down after the finish line, not before it. It was no surprise. My muscles were contracting, I had cramp and I had pain all over my body."

The dramatic scenes will be of concern to organisers who have set up special mist stations to cool athletes and issued warnings about the extreme heat.

Many races have ended with exhausted athletes motionless on the tracks, a poor advertisement for the sport in front of a worldwide audience.

When they get up, their first comments are invariably about the heat.

"It's very hot, that's the most difficult thing," said men's 3,000m steeplechaser Roba Gary. "We've prepared very well for this, but the weather is a problem -- the heat and humidity."

Belarussian runner Volha Krautsova dropped out of the women's 10,000m with apparent exhaustion, while athletes are constantly dousing themselves in iced water.

Spectators meanwhile have been fending off the heat, which does not let up even during the evening, with thousands of hand-held fans, creating a surreal fluttering vista across the Nagai Stadium stands.

The bad news is it is set to continue with Sunday tipping the mercury at 36 C and similar forecasts for Monday and Tuesday.

The International Association of Athletics Federations (IAAF) warned athletes to spend plenty of time acclimatising, drink lots of fluids, avoid sunburn and wear thin socks.

"Certainly, we have been monitoring the weather conditions very closely and we are working with the medical people on that," said IAAF competitions director Paul Hardy.

"The organising committee is making more provisions than normal, encouraging use of more sponges and water. And after races today, once again we had a quick debriefing and we'll implement further provisions for further races."

But the heat is at least popular with the sprinters, who can warm up more easily and are less likely to injure themselves.

"I'm loving the heat. I live in Fayetteville, Arkansas where it's always like this," said American Wallace Spearmon, the reigning 200m silver medallist.

"I don't mind it at all."