Pilgrims pray at dawn on Mount Arafat on Saturday.

More than 1.5 million Muslims will pray on Mount Arafat in soaring temperatures on Saturday, in the high-point and most gruelling day of the annual Hajj pilgrimage.

Worshippers from all over the world will climb the rocky, 70-metre (230-feet) hill, about 20 kilometres from the Holy City of Makkah, where the Prophet Mohammad (PBUH) is believed to have given his last sermon.

The desert summer heat is expected to hit 43˚C, creating challenges especially among the elderly during a day of prayer and reciting the Holy Quran.

The Hajj, which takes at least five days to complete and is mostly outdoors, "is not easy because it is very hot," said Abraman Hawa, 26, from Ghana.

"We have sun... but it is not as hot. But I will pray to Allah at Arafat, because I need His support," she added.

Saudi authorities have urged pilgrims to drink plenty of water and protect themselves from the sun. Since men are prohibited from wearing hats, many carry umbrellas.

More than 10,000 heat-related illnesses were recorded last year, 10 per cent of them heat stroke, a Saudi official told AFP this week.

The Hajj, one of the world's biggest religious gatherings, is increasingly affected by climate change, according to a Saudi study that said regional temperatures were rising 0.4˚C each decade. But Mohammed Farouk, a 60-year-old Pakistani pilgrim, was not put off by the Gulf kingdom's scorching summer sun. The Hajj is "very important for me as a Muslim," he said.

The enormous crowds of worshippers spent the night in a giant tented city in Mina, a valley several kilometres outside the Holy City of Makkah.

After Arafat they will head to Muzdalifah, where they will collect pebbles to carry out the symbolic "stoning of the devil" ritual in Mina on Sunday.

The Hajj is said to follow the path of the Prophet Mohammad's (PBUH) final pilgrimage, about 1,400 years ago.

The kingdom received more than 1.8 million pilgrims last year for the Hajj, around 90 per cent of whom came from abroad.

It also welcomed 13.5 million Muslims who came to perform Umrah, the small pilgrimage that can be done all year round, and aims to reach 30 million by 2030.

Agence France-Presse