Several days of extraordinary heat took a major toll on one of the world’s largest religious gatherings, with hundreds reported dead as part of this year’s Hajj pilgrimage in the desert of Saudi Arabia.Two unnamed Arab diplomats told the Agence France-Presse news service that 323 people from Egypt alone had died, most because of heat-related illnesses. Egypt has not yet shared an official count, but other countries whose citizens flocked to the holy city of Mecca have been reporting tolls: at least 138 from Indonesia, 41 from Jordan and 35 from Tunisia.The Associated Press, reporting from Mecca, also cited a triple-digit death toll, and described people lining up at an emergency health facility to get information about missing loved ones.While it was not clear how many of those deaths were attributable to heat, this year’s Hajj coincided with a heat wave unusually searing for June. At one point during the multiday gathering, temperatures reached 51.8 degrees Celsius, or 125 Fahrenheit. More than 1.8 million people took part in the pilgrimage.“The numbers were huge. … We could not breathe,” said 37-year-old Ahmad Bahaa, an engineer from Egypt who works in Saudi Arabia and took part in the Hajj.He described “shocking scenes” in Mecca as the heat took hold, especially among unregistered pilgrims who lack permits and are barred from accessing services and facilities, including air-conditioned tents.Every year, Saudi Arabia approves a set number of pilgrim visas, with quotas for each Muslim-majority country. But hundreds of thousands of people who don’t secure visas still manage to participate.“Ambulances were moving nonstop, collecting people left and right,” Bahaa said. “People were sleeping on the sidewalks. … I saw someone right in front of our tent who collapsed and could not even move.”Mecca is Islam’s holiest city, the birthplace of the prophet Muhammad, and the Hajj is one of the five pillars of Islam — a ritual to be completed at least once in the lifetime of every Muslim with the means. But Mecca is also an inland city hit by humid air from the Red Sea. Many who make the pilgrimage are elderly. People pack into tight areas. Over about five days, they can spend several dozen hours outdoors.The outcome — in this instance — shows how mass-scale outdoor gatherings have the potential to become more lethal as parts of the world warm beyond what humans can withstand.Over the years, Saudi Arabia has taken steps to reduce the dangers, erecting more than 100,000 air-conditioned tents, distributing water and umbrellas, planting trees and preparing facilities to respond to heat-related illnesses. Still, a paper published this year by Saudi-based researchers said that while these measures had been helpful, “concerns arise about the sufficiency of current mitigation measures in the face of escalating heat.”In Mecca, as in much of the world, the number of dangerously hot days is surging. By 2050, Mecca will have an estimated 182 days of dangerous heat for those outdoors in the sun, according to an analysis conducted last year by The Washington Post and the nonprofit CarbonPlan. Just as notable, it will have an estimated 54 days when heat is dangerous even in the shade, compared with almost zero at the turn of this century. By those measures, Mecca will be one of the least hospitable places on Earth.A 2019 study of extreme heat during the Hajj indicated “a significant warming trend during the last 30 years of close to 2° C,” higher than the global average, which it attributed to human-caused warming.“I have not experienced any heat like this before. It was very strenuous,” said Adonis Imam, a physician from Augusta, Ga., who was in Mecca. “We got exhausted pretty quickly. Even short walks would take a toll on us.” Imam, 36, said that Saudi misting and cooling measures helped, and groups were advised not to be outdoors between noon and the late afternoon.Those with no cool place to stay scrambled for spots of shade. “They were spraying water all over their bodies, holding umbrellas, drinking lots of water,” said Alaa al-Din Mohey, 33. The days he spent in Mecca, he said, were the hottest of his life.The dates of the Hajj are determined by the shorter lunar calendar, meaning the mass-scale gathering rotates gradually across all seasons. Last year, when it was held slightly deeper into June, thousands of people were treated for heat exhaustion.The 2019 paper noted that heat stress levels are projected to grow when the Hajj cycles again into the hottest months, between 2047 and 2052.In 1985, a previous point when the Hajj was held under sweltering conditions, more than 1,000 people died of heat stroke, according to a study in the Annals of Saudi Medicine.Saudi Arabia has not mentioned any death toll, though earlier its state news agency reported several thousand cases of heat stress and sunstroke. On Wednesday, its state news agency said the Hajj had been a “success,” citing an effective execution of “all plans related to security, prevention, organization, health, services, and traffic management.”Harlan reported from Rome and Mahfouz from Cairo. Sarah Dadouch in Beirut contributed to this report.