At least 717 people have been crushed to death and around 805 hurt in a stampede of pilgrims in one of the worst incidents in years to hit the Muslim Hajj in Saudi Arabia.
Saudi Arabia's civil defence service said rescue operations were under way after the stampede in Mina, where almost two million pilgrims were taking part in the last major rite of the Hajj.
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, placing victims on stretchers and desperately trying to resuscitate others.
It was not immediately clear what had caused the stampede.
Pilgrims had converged on Mina just outside Mecca on Thursday to throw pebbles at one of three walls representing Satan, the symbolic 'stoning of the devil' that marks the last day of the event.
The world's 1.5 billion Muslims were marking Eid al-Adha, the Feast of Sacrifice, the most important holiday of the Islamic calendar.
The Hajj is among the five pillars of Islam and every capable Muslim must perform it at least once in a lifetime.
In the past, the pilgrimage was for years marred by stampedes and fires, but it had been largely incident-free for nearly a decade following safety improvements.
Preparations for this year's Hajj were marred when on September 11 a construction crane collapsed at Mecca's Grand Mosque, Islam's holiest site, killing 109 people.
In January 2006, 364 pilgrims were killed in a stampede during the stoning ritual in Mina.
Thursday's ritual was taking place at a five-story structure known as the Jamarat Bridge, which cost more than $1billion to build and was used during earlier pilgrimages.
Almost one kilometre long, it resembles a parking garage and allows 300,000 pilgrims an hour to carry out the ritual.
The faithful had gathered until dawn Thursday at nearby Muzdalifah where they chose their pebbles and stored them in empty water bottles.
It was not immediately clear if the stoning ritual at Mina would continue as planned until Saturday after the stampede.
Yesterday, they had spent a day of prayer on a vast Saudi plain and Mount Arafat, a rocky hill about 10 kilometres from Mina, for the peak of the Hajj pilgrimage.
The flow of exhausted pilgrims was so big that Saudi security forces had to form a human chain along the roads of the vast Arafat plain while a jets of water were sprayed on the huge crowds to keep them cool amid searing heat.
Many of the faithful from around the globe camped at the foot of Mount Arafat where they slept and prayed – despite the scorching sun – at the spot where Prophet Mohammed is believed to have given his final sermon.
Carrying colourful umbrellas, they walked from dawn in massive crowds towards the slippery, rocky hill which is also known as Mount Mercy.
Here they believe Mohammed gave his final sermon 14 centuries ago after leading his followers on Hajj.
Many reached Arafat by bus while some walked from the holy city of Mecca about nine miles away. Along the way, volunteers handed out boxes of food and cold water bottles.
For many pilgrims, Hajj is the spiritual highlight of their lives.
'We feel blessed. I got goosebumps, a feeling that cannot be explained, when reaching the top of the mountain,' said Ruhaima Emma, a 26-year-old Filipina pilgrim, who said she has been 'praying for a good life for everyone'.
For Akram Ghannam, 45, from war-torn Syria, being in Arafat is a 'feeling that cannot be described. I pray to God for the victory of all those who are oppressed.'
Other pilgrims arrived from nearby Mina using the elevated Mashair Railway linking the holy sites of Arafat, Muzdalifah and Mina, a tent city where many pilgrims spent Tuesday night.
This year's gathering is about the same size as last year's, with 1.4 million foreign pilgrims joining hundreds of thousands of Saudis and residents of the kingdom.
Islam requires all able-bodied Muslims to perform the Hajj once in a lifetime – and each year huge crowds are drawn to Mecca to carry out a series of rituals and prayers aimed at erasing past sins.
Pictures yesterday show newly-arrived pilgrims circling the Kaaba, the black cube-shaped structure in Mecca's Grand Mosque toward which all Muslims pray.
Saudi authorities deploy 100,000 police and vast numbers of stewards to ensure safety and help those who lose their way. They have also invested in expensive infrastructure, including a new urban railway, to ferry pilgrims around safely.
It comes just weeks after a crane toppled into Mecca's Grand Mosque killing more than 100 people and injuring hundreds more.
The crane which collapsed during a thunderstorm was one of several working on a multi-billion-dollar expansion of the mosque to accommodate mounting numbers of faithful.
Despite the tragedy, Saudi officials quickly vowed that the Hajj to Mecca would go ahead with thousands descending on the city.
During the Hajj, pilgrims shed symbols of worldly materialism, entering a state known as 'ihram' - women forgo makeup and perfume for loose fitted clothing and a head-covering while men wear seamless terry cloth white garments.
The pilgrimage is among the five main pillars of Islam, which also include belief in the oneness of God and the Prophet Muhammad as his final messenger, five daily prayers facing toward the Kaaba, annual charity and fasting during the month of Ramadan.
Muslims believe the Hajj traces the paths of the Prophets Abraham, Ishmael and Muhammad. Pilgrims start the Hajj in Mecca, before heading to the nearby tent city of Mina, five kilometers (three miles) away.
In Mecca, they circle the Kaaba counterclockwise seven times, and also re-enact the path of Hagar, the wife of the patriarch Abraham, who Muslims believe ran between two hills searching for water for her dying young son.
Tradition holds that God then brought forth a spring of water that runs until this day and which Muslims drink from during the Hajj.
Since arriving to Mecca over the past several weeks, hundreds of thousands have chanted, 'Labayk Allahuma Labayk,' or 'Here I am, God, answering your call. Here I am.'
Many prayed for their deceased kin. Sadi Zawya, who lost his wife earlier year and is now raising his five children on his own was among the sea of people.
Arriving from Egypt, 62-year-old Hoda Darahim said her 35 year-old daughter died this year, and that she is now raising her two grandchildren, relying on financial help from her older sons and the small government pension she receives.
'Her dream was to perform the Hajj,' Darahim said of her daughter. 'So I am fulfilling her wish.'
Moussa bin Abdullah Butu, a 38-year-old artist from Nigeria, said this will be his third Hajj but an especially challenging one since he lost his two-year old son, Abdullah, to a long illness earlier this year. 'In the white ihram, the rich man and the poor, we are all together,' he said.
The Hajj requires physical perseverance, finances and a coveted Hajj visa, which is limited to country-by-country quotas to manage crowd safety. Butu said he was very grateful for his visa and the opportunity to once again perform the hajj.
'I know that I am one of the people that Allah chose ... I am one of the people Allah called this year,' Butu said.
The main day of Hajj this year falls on Wednesday, when between 2 to 3 million pilgrims gather in a valley called Arafat, packed shoulder to shoulder in prayer.
It is at Arafat where the Prophet Muhammad delivered his final sermon some 1,400 years ago, calling for equality and unity among Muslims.
