Tens of thousands of Muslim pilgrims clambered up rocks and nestled between boulders on Thursday as they embarked on a day of worship under a scorching sun at the Hajj in Saudi Arabia.

Braving the heat and uneven surfaces, they scaled Mount Arafat to perform a key ritual of the annual Islamic pilgrimage.

They opened their arms in prayer, smartphones and hearts raised, as outsized fans sprayed fine mists of water to cool them during their long hours of worship. Pilgrims must remain in Arafat until sunset. Temperatures exceeded 40 C.

Away from the worship, an ice cream kiosk nearby provided some relief.

Salem Naji Ahmed, from Yemen, traveled four days overland to neighbouring Saudi Arabia because he couldn't afford the flight.

"These sacred feelings are the most luxurious and best thing about Arafat," he told The Associated Press. "We came from Yemen, four days and nights. It was very tiring, but we came to please Allah and perform one of his obligations in the holy sites and in the best area on earth."

The rocky hill holds immense significance in Islam. Arafat is mentioned in the Qur'an and it is where the Prophet Muhammad is said to have given his last sermon on his final Hajj.

According to traditional sayings of the prophet, the Day of Arafat is the most sacred day of the year, when God draws near to the faithful and forgives their sins.

Ibrahim Mahmoud, from Egypt, said the heat was affecting the elderly. "Elderly pilgrims are getting fatigued because of the heat. Some are not getting out due to the high temperatures and cannot walk long distances."

The Health Ministry said Thursday there had been a 90 per cent decrease in heat-stress cases compared to last year, when more than 1,300 people died amid high temperatures at the holy sites.

Hajj is one of the Five Pillars of Islam, a religious obligation involving rituals and acts of worship that every Muslim must fulfil if they have the money and are physically able to do it. Saudi authorities have not disclosed the total number of pilgrims at this year's Hajj.

Earlier, they said more than 1.5 million foreigners were participating. Last year, there were 1,611,310 pilgrims from outside the country.