The ultra-conservative kingdom, which has launched sweeping social and economic transformation programmes under the helm of Crown Prince Mohammed bin Salman, has mobilised vast resources for the six-day journey.
Every Muslim is required to complete the hajj, one of the five pillars of Islam, at least once in their lifetime if they have the means to do so.
Tens of thousands of security personnel have been deployed for the pilgrimage, which was struck by its worst ever disaster three years ago when around 2,300 worshippers were crushed to death in a stampede.
The Saudis have also launched a "smart hajj" initiative this year, with apps to help pilgrims with everything from travel plans to medical care.
The interior ministry said on Saturday the number of pilgrims arriving in Mecca had already surpassed the two million mark, mostly from abroad including large contingents from Egypt, India, Pakistan and Bangladesh.
Most of the pilgrims began moving on Sunday from Mecca to the nearby Mina valley where they will spend the night in fire-resistant tents.
On Monday, pilgrims will climb nearby Mount Arafat for the climax of the hajj, praying and reading the Qu'ran.
The Prophet Mohammed delivered his final sermon from Mount Arafat to Muslims who had accompanied him on his final hajj, according to Islam.
After sunset, pilgrims head to Muzdalifah, half-way between Arafat and Mina, where they stay at least until midnight.
They gather pebbles to perform the symbolic stoning of the devil on the eve of the Eid al-Adha feast, which marks the end of hajj.
This year's pilgrimage comes with the oil-flush kingdom witnessing unprecedented change and rights campaigners expressing alarm about a crackdown on dissent.
Critics have also charged Riyadh with politicising the religious rite. On Saturday, Qatar accused Saudi Arabia of barring its citizens from this year's hajj pilgrimage amid a year-long blockade imposed on the Gulf state by a Saudi-led coalition.
Around 1,200 Qataris are eligible to perform hajj under a quota system but Qatar says it has become impossible to obtain permits.
Agencies contributed to this report.
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