The Hajj, which is expected to draw more than two million pilgrims this year, represents a key rite of passage for Muslims and a massive logistical challenge for Saudi authorities - as these stunning images show.
Saudi Arabia has said that more than 2.3 million pilgrims, a lot of them from outside Saudi Arabia, have arrived for the five-day ritual, a religious duty once in a lifetime for every able-bodied Muslim who can afford the journey.
From first light on Sunday, throngs of the faithful have circled the cube-shaped Kaaba in Mecca, one of the holiest sites in Islam. Nearly 14,000 worldwide and domestic flights have so far transported pilgrims with around 21,000 buses also used.
Some pilgrims carried umbrellas to protect themselves from the sun as temperatures surpassed 40 degrees Celsius after an evening of thunderstorms and high winds.
Tens of thousands of security personnel have been deployed for the pilgrimage, which was struck three years ago by what was termed the worst ever disaster when around 2,300 worshippers were crushed to death in a stampede.
Pilgrims pour out of cars stuck in bumper-to-bumper traffic, opting for a more brisk walking pace instead of a one tyre rotation every minute. For the first time this year, Indian women will be going on Haj without the company of a male relative.
"For many Muslims this is the big, the biggest, dream of life, to see Kaaba and pray for yourself and the whole Muslim nation".
"I knew it would be a little hard to climb Mount Arafat", said Nigerian pilgrim Saidou Boureima.
Jai Saleem, a 37-year-old Pakistani, said he cried when he and his wife arrived on Mount Arafat, where Muslims believe Prophet Mohammed delivered his final sermon.
Pilgrimage is also the backbone of a plan to expand tourism under a drive to diversify the kingdom's economy away from oil. Around the world, Muslims will mark the end of hajj with a celebration called Eid al-Adha.
Qatar has been blockaded since June 2017 by Saudi Arabia and its allies.
Riyadh has banned all flights to and from Doha and has imposed a blockade on Qatar.
"There is no chance this year for Qatari citizens and residents to travel for hajj", the committee's Abdullah al-Kaabi said. "But being here I feel comfort and peace and I pray to God for the war to end".