Some two million Muslims poured out of the holy city of Makkah yesterday to begin the annual hajj, the world's largest annual assembly.
The hajj rituals will peak today.
The pilgrims moved from Makkah to nearby Mina valley by road, by train or on foot, the men wearing ihram, the seamless two-piece white garment that rituals require, the women covered up except for their faces and hands.
In Mina, a small site with 45,000 fire-resistant tents that can accommodate two million people, they will pray and rest before moving on to Mount Arafat today.
The recently constructed electric railway is scheduled to carry 400,000 of the pilgrims.
Saudi Arabia has deployed more than 100,000 troops to ensure the safety of the pilgrims and has warned it will tolerate no demonstrations or disturbances.
Interior ministry spokesman General Mansur al-Turki told a news conference on Saturday that the kingdom "will not allow any action that may subject the pilgrims' safety to risk".
Security forces are monitoring the holy sites in and around Makkah with 4,200 hi-tech cameras, some of which are so advanced that can cover a distance of 60 kilometres.
Authorities said more than 20,000 buses are ferrying the pilgrims to Mina using 58 tunnels under Makkah's high mountains.
But some pilgrims, like Egyptian Mustafa Abu el-Wafa, decided to walk the 10-kilometre distance despite the heat and humidity.
"I am so excited that I finally managed to perform the hajj and come to the areas that the Prophet (Mohammed) had once been to," Wafa said as he walked towards Mina.
From Mina, pilgrims will head to nearby Mount Arafat, where they spend today in prayer until sunset.
The oil-rich kingdom has also mobilised huge medical and civil defence resources to ensure the smooth movement of the pilgrims, around 1.4 million of whom come from abroad.
That figure is sharply down on last year's 1.75 million.
Riyadh has imposed a 20-percent cut this year on the quota for pilgrims coming from abroad.
It has also slashed the number of domestic pilgrims by half because of MERS virus fears and reduced capacity resulting from multi-billion-dollar construction work.
The hajj is one of the five pillars of Islam that every capable Muslim must perform at least once.