At least 717 pilgrims from around the world have been killed in a crush outside the Muslim holy city of Mecca, in the worst disaster to strike the annual Hajj pilgrimage for 25 years.
At least 863 others were injured at Mina, a few kilometres east of Mecca, when two large groups of pilgrims arrived together at a crossroads on their way to performing the "stoning of the devil" ritual at Jamarat, Saudi civil defence said.
Thursday's disaster was the worst to occur at the pilgrimage since July 1990, when 1426 pilgrims suffocated in a tunnel near Mecca. Both incidents occurred on Eid al-Adha (Feast of the Sacrifice), Islam's most important feast and the day of the stoning ritual.
The Hajj, the world's largest annual gathering of people, has been the scene of numerous deadly stampedes, fires and riots in the past, but their frequency has been greatly reduced in recent years as the government spent billions of dollars upgrading and expanding Hajj infrastructure and crowd control technology.
Street 204, where the crush occurred, is one of two main arteries leading through the camp at Mina to Jamarat, the site where pilgrims ritually stone the devil by hurling pebbles at three large pillars. In 2006, at least 346 pilgrims died in a stampede at Jamarat.
"Work is under way to separate large groups of people and direct pilgrims to alternative routes," the Saudi Civil Defence said.
It said more than 220 ambulances and 4,000 rescue workers had been sent in to help the injured. Some of the wounded were evacuated by helicopters.
Efforts to improve safety at Jamarat have included enlarging the three pillars and constructing a three-decker bridge around them to increase the area and number of entry and exit points for pilgrims to perform the ritual.
More than 100,000 police and thousands of video cameras are also deployed to allow groups to be dispersed before they reach dangerous levels of density.
"Please pilgrims do not push one another. Please leave from the exit and don't come back by the same route," an officer kept repeating through a loudspeaker at Jamarat.
Two weeks ago 110 people died in Mecca's Grand Mosque when a crane working on an expansion project collapsed during a storm and toppled off the roof into the main courtyard, crushing pilgrims underneath.
Saudi Arabia's Crown Prince and Interior Minister Prince Mohammed bin Nayef bin Abdelaziz ordered a committee to be formed to investigate the disaster and present its findings to King Salman, the Interior Ministry said.
The ministry spokesman, Mansour Turki said the investigation would look into what caused an unusual density of pilgrims to congregate at the location of the disaster. "The reason for that is not known yet," he told a news conference in Mina.
UN Secretary General Ban Ki-moon and the White House offered condolences over the deaths.
"This tragic incident is all the more distressing as it took place on the first day of the Holy Eid Al-Adha marking the end of the annual Hajj season," the secretary general's spokesman said in a statement.
New Zealand's Ministry of Foreign Affairs says no New Zealanders were involved in the fatal stampede. The embassy in Riyadh was in close contact with Saudi authorities, a spokesperson said.
In Washington, White House spokesman Ned Price said: "The United States expresses its deepest condolences to the families of the hundreds of Hajj pilgrims killed and hundreds more injured in the heartbreaking stampede in Mina, Kingdom of Saudi Arabia."
Iranian state news agency IRNA said at least 95 Iranians were among the dead and quoted Iran's Deputy Foreign Minister Hossein Amir Abdollahian as saying Saudi Arabia was responsible.
The semi-official Fars news agency reported that Tehran summoned the Saudi charge d'affaires to lodge an official complaint over the disaster.
South African Acting President Cyril Ramaphosa extended condolences to families of the victims and said his government was awaiting information about his country's pilgrims.