At least 453 people taking part in the Hajj pilgrimage have been killed in a stampede near the Islamic holy city of Mecca, officials in Saudi Arabia say.
Another 719 people were injured in the incident at Mina, which occurred as two million pilgrims were taking part in the Hajj's last major rite.
They converge on Mina to throw stones at pillars representing the devil.
Preparations for the Hajj were marred when a crane collapsed at Mecca's Grand Mosque this month, killing 109 people.
The Saudi civil defence directorate said on Twitter that 4,000 personnel had been sent to the scene of Thursday's stampede, along with more than 220 emergency and rescue units. The injured are being taken to four hospitals in the area.
BBC Hausa Service correspondent Tchima Illa Issoufou, who was with some of those affected in Mina, said there were many pilgrims from Niger among the dead.
The UK Foreign Office said it was in contact with the local authorities and was urgently seeking more information about whether British nationals were involved.
The civil defence directorate did not say what caused the stampede.
However, Al Jazeera English's Basma Atassi in Mina reported that it took place among tents erected on Street 204, and not where people were throwing stones at the Jamarat pillars - which are surrounded by a five-storey bridge-like structure.
"There were a group of pilgrims sitting on the floor. Another group walked over to them and the stampede took place," she said.
Mina, a large valley about 5 km (3 miles) from Mecca, also houses more than 160,000 tents where pilgrims spend the night during the pilgrimage.
The stampede was the deadliest at the Hajj since 2006, when more than 360 pilgrims were killed in the same area.
The Saudi authorities have been working on improving transport and other infrastructure in the area in an attempt to try to prevent such incidents.
The Hajj is the fifth and final pillar of Islam. It is the journey that every able-bodied adult Muslim must undertake at least once in their lives if they can afford it.