At least 717 Muslim worshippers have died in a stampede at the annual Hajj pilgrimage, according to Saudi Arabian authorities.
The tragedy has also left up to 805 pilgrims injured.
The incident took place at Mina, on the outskirts of Mecca, where pilgrims go to take part in a ritual 'stoning of the devil'. Two groups arrived together at a crossroads.
Fatal stampedes have taken place before there, with the worst disaster taking place in 1990 when 1,426 pilgrims were crushed to death in a tunnel. Authorities have tried to take measure to improve safety for the 2 million pilgrims.
However, reports saw that a few pilgrims fell when they were crossing a bridge that acts as a bottleneck and others tried to escape the press.
Video posted on Twitter showed bodies, clad in the white toweling of those undertaking haj, lying on the ground by the side of the road, surrounded by debris, as pilgrims and rescue workers attempted to revive them.
"Work is under way to separate large groups of people and direct pilgrims to alternative routes," the Saudi Civil Defence authority said on its Twitter account.
It said more than 220 ambulances and 4,000 rescue workers had been sent to the stampede's location to help the injured. Saudi-owned Al-Arabiya television channel showed a convoy of ambulances driving through the Mina camp. Some of the wounded were evacuated by helicopters.
Brigadier Mansour al-Turki blamed the deaths on "the mass of people on the bridge, and the state of panic that erupted after some of them fell".
"The trampling begins with a small number of people falling, followed by a state of panic and an attempt to escape the crowd, which leads to an increasing number of victims," he said.
The incident is the second to have overshadowed the Hajj this year. A crane used in a huge building project aimed at expanding the capacity of the Grand Mosque collapsed and killed 110 people.
Additional reporting by Reuters.