More than 1.5 million Muslims have gathered on Mount Arafat in Saudi Arabia to pray and recite the Koran during the high-point and most grueling day of the annual Hajj pilgrimage.
The temperature is expected to reach 43 degrees Celsius (109.4 degrees Fahrenheit), posing challenges for the elderly and others in the desert heat.
Pilgrims from around the world have climbed the rocky, 70-metre (230-feet) hill, where the Prophet Mohammed is believed to have given his last sermon.
Saudi authorities have urged pilgrims to drink plenty of water and protect themselves from the sun, as the Hajj, which is mostly outdoors, is not easy due to the heat.
The Hajj is one of the world's biggest religious gatherings and is increasingly affected by climate change, with temperatures rising 0.4C each decade in the region.
Despite the heat, many pilgrims are determined to complete the pilgrimage, which is important for them as Muslims.
After spending the night in a giant tented city in Mina, pilgrims will head to Muzdalifah to collect pebbles for the symbolic "stoning of the devil" ritual in Mina on Sunday.
The Hajj follows the path of the Prophet Mohammed's final pilgrimage and is an important source of legitimacy for the Al Saud dynasty.
The Hajj is also a major financial windfall for Saudi Arabia, which is trying to develop religious tourism as part of a drive to reduce its dependence on crude oil.
The kingdom received more than 1.8 million pilgrims last year for the Hajj and aims to reach 30 million by 2030 for Umrah, the small pilgrimage that can be done all year round.
This year's Hajj takes place in the shadow of the Gaza war, after eight months of bloodshed that is an open wound for many in the Muslim world.
