MINA, Saudi Arabia » A horrific stampede killed at least 717 pilgrims and injured hundreds more Thursday on the outskirts of the holy city of Mecca in Saudi Arabia, the deadliest tragedy to strike the annual hajj pilgrimage in more than two decades.

At least 863 pilgrims were injured in the crush, said the Saudi civil defense directorate, which provided the death toll. The tragedy struck as Muslims around the world marked the start of the Eid al-Adha holiday.

It was the second major disaster during this year’s hajj season, raising questions about the adequacy of measures put in place by Saudi authorities to ensure the safety of the roughly 2 million Muslims taking part in the pilgrimage. A crane collapse in Mecca nearly two weeks earlier left 111 people dead.

Many of the victims were crushed and trampled to death as they were on their way to perform a symbolic stoning of the devil by throwing pebbles against three stone columns in Mina, a large valley about 3 miles from Mecca that has been the site of hajj stampedes in past years. The area houses more than 160,000 tents where pilgrims spend the night during the pilgrimage.

Two survivors interviewed by The Associated Press said the disaster began when one wave of pilgrims found themselves heading into a mass of people going in another direction.

“I saw someone trip over someone in a wheelchair and several people tripping over him. People were climbing over one another just to breathe,” said one of the survivors, Abdullah Lotfy, 44, from Egypt. “It was like a wave. You go forward and suddenly you go back.”

Lotfy said that having two flows of pilgrims interacting in this way should never have happened. “There was no preparation. What happened was more than they were ready for,” he said of the Saudi authorities.

Later Thursday, King Salman expressed his condolences over the tragic crush and pledged a speedy investigation. He said he has asked for a review of “all existing plans and arrangements … to improve the level of organization and management of the movement” of pilgrims at the hajj.

Saudi Arabia takes great pride in its role as the caretaker of Islam’s holiest sites and host to millions of pilgrims annually. But the hajj poses an immense logistical and security challenge for the kingdom, given the sheer number of hundreds of thousands of people — from differing linguistic and cultural backgrounds, many of whom have saved for years for the once-in-a-lifetime opportunity to make the hajj — intent on following the same set of rituals at about the same time.

The kingdom’s Interior Ministry said later Thursday that the crush appears to have been caused by two waves of pilgrims meeting at an intersection.

The ministry’s spokesman, Maj. Gen. Mansour al-Turki, said high temperatures and the fatigue of the pilgrims may also have been factors in the disaster. He said there was no indication that authorities were to blame for the event, adding that “unfortunately, these incidents happen in a moment.”

Another survivor, Ismail Hamba, 58, from Nigeria, recalled falling down and then being trampled over by marching pilgrims. “It was terrible, it was really, really terrible,” he said.

Thursday’s tragedy struck during a morning surge of pilgrims at the intersection of streets 204 and 223 as the faithful were making their way toward a large structure overlooking the columns, according to the civil defense directorate.

The multi-story structure, known as Jamarat Bridge, is designed to ease the pressure of the crowds and prevent pilgrims from being trampled.

Ambulance sirens blared and helicopters hovered overhead as rescue crews rushed the injured to nearby hospitals. More than 220 rescue vehicles and some 4,000 members of the emergency services were deployed soon after the stampede to try to ease the congestion and provide alternative exit routes, according to the directorate.

Amateur video shared on social media showed a horrific scene, with scores of bodies — the men dressed in the simple terry cloth garments worn during hajj — lying amid crushed wheelchairs and water bottles along a sunbaked street.

Survivors assessed the scene from the top of roadside stalls near white tents as rescue workers in orange and yellow vests combed the area.

International media covering the hajj, including The Associated Press journalists in Mina, were restricted from visiting the site of the accident for several hours and from immediately leaving an Information Ministry complex where the press is housed during the final three days of the pilgrimage, per government rules.

Photos released by the directorate on its official Twitter account showed rescue workers helping the wounded onto stretchers and loading them onto ambulances near some of the tents.

Dozens of bodies could still be seen in the streets at dusk despite the presence of ambulances and refrigerator trucks to haul away the dead.

Saudi authorities take extensive precautions to ensure the security and the safety of pilgrims during the hajj, which is an obligation for every able-bodied Muslim. The pilgrimage began in earnest Tuesday. There are about 100,000 security forces deployed this year to oversee crowd management and ensure pilgrims’ safety during the five-day pilgrimage.

At Mina specifically, authorities have put measures in place over the years to try to alleviate the pressure posed by masses of pilgrims converging on the site of the stoning ritual.

Officials use surveillance cameras and other equipment to limit the number of people converging on the site, and the Jamarat Bridge has multiple exits to facilitate the flow of people.

But tragedies are not uncommon.

The death toll from Thursday’s crush far exceeded that of a similar incident in 2006, near the same site, when more than 360 pilgrims were killed in a stampede. Another stampede at Mina in 2004 left 244 pilgrims dead and hundreds injured.

The deadliest hajj-related tragedy happened in 1990, when at least 1,426 pilgrims perished in a stampede in an overcrowded pedestrian tunnel leading to holy sites in Mecca.

The latest tragedy is certain to have touched many different countries as the victims likely included pilgrims of different nationalities.

At least 95 Iranian pilgrims perished, according to the official IRNA news agency. Deputy foreign minister, Hossesin Amir Abdollahian, said his ministry summoned the Saudi envoy to Tehran for an official protest over what he called the “inadequate performance of Saudi authorities” in the incident.

Later Thursday, Iran’s Supreme Leader Ayatollah Ali Khamenei urged Saudi authorities to accept their responsibility for the tragedy, which he said was caused by “mismanagement.” Khamenei also announced a three-day mourning period in Iran.

The United States expressed its deepest condolences for the victims of the “heartbreaking stampede” outside Mecca. National Security Council spokesman Ned Price said the U.S. joins in mourning for “the tragic loss of these faithful pilgrims.”

The U.N. Secretary-General Ban Ki-moon was “deeply saddened” to hear of the deaths, his spokesman said in a statement.

In the Pakistani city of Lahore, Sajida Arif, said her father, Haji Arif, died in the stampede. “Before leaving for the hajj, he told me he had a wish to be buried in Mecca,” she said.

Less than two weeks ago, a giant construction crane came crashing down on the Grand Mosque in Mecca, the focal point of the hajj. The Sept. 11 accident killed at least 111 people and injured more than 390.

Authorities blamed the crane collapse on high winds during an unusually powerful storm, and faulted the construction giant Saudi Binladin Group, which oversees construction at the mosque, for not following operating procedures.

Schreck reported from Dubai, United Arab Emirates. Associated Press writers Sarah El Deeb in Beirut; Munir Ahmed in Islamabad; Nasser Karimi in Tehran, Iran, and Merrit Kennedy in Cairo contributed to this report.