Over two million faithful have started gathering at the tent valley of Mina as the Haj rituals begin on Wednesday.
There, they perform a ritual walk seven times around the Kaaba, a black masonry cube wrapped in a heavy silk cloth embroidered with Koranic verses in golden embroidery.
The shrine is the point towards which Muslims around the world pray.
Pilgrims then head for Mina, five kilometers (three miles) further east, where hundreds of thousands of people will gather before setting off on Thursday at dawn to climb Mount Arafat, the pinnacle of the pilgrimage.
Saudi authorities have placed misting fans on the esplanade of the Grand Mosque to take the edge off the intense heat.
On the eve of the first rites of the pilgrimage, the walkways thronged with people and the smell of musk wafted through the air.
Sitting in the shade of trees or reinforced concrete bridges, the faithful waited patiently for the next call to prayer. Others continued their march, protected by a prayer mat or a small umbrella fixed on the head with an elastic band.
Several times throughout the day, well-run teams of employees, mostly Asian, cleaned the esplanade with jets of water.