A pillar of the Islamic faith, hajj this year comes against the backdrop of widespread revulsion among Muslims towards Islamic State group jihadists.
Saudi Arabia and four other Arab nations have joined Washington in air strikes in Syria against the militants, who have declared a "caliphate" straddling that country and Iraq where they have committed a spate of atrocities.
Saudi authorities say close to 1.4 million believers have come from 163 nations for hajj, the world's biggest Muslim gathering.
They are following the 1,400-year-old tradition of Prophet Mohammed, alongside pilgrims from Saudi Arabia.
The faithful streamed out of the holy city of Mecca heading for the nearby Mina valley, where most would wait until the early hours of Friday before gathering at the hill known as Mount Arafat for the peak of hajj, a day of prayer and recitations from the Koran.
Late Thursday some had already reached Mount Arafat. Among them was Iman Izzidine, 42, who has fled Syria's civil war.
"I thank God for this indescribable feeling" of being in the Muslim holy land, said the soft-spoken woman dressed in black from head to toe.
"I will pray to God for victory and for Syria to return better than before," she said, her voice breaking and tears welling in her brown eyes.
Mount Arafat is where Prophet Mohammed gave his final sermon after leading his followers on the hajj.
Hundreds of pilgrims arriving at Arafat carried suitcases and other luggage among thousands of white tents which stood ready on a vast plain to provide temporary homes for the multitude.
Elderly pilgrims sat in wheelchairs, little ones in prams, and others leaned on sticks as they made their way towards their camps.
Men arrived wearing a seamless two-piece white garment, ihram, symbolising a state of purity and emphasising their unity regardless of social status or nationality.
Women also generally wear white, exposing only their faces and hands.
Many pilgrims set up their own colourful tents as small children ran about, the boys in ihram and girls wearing veils, while others simply placed rugs among the shrubs and slept.
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- Commandos, helicopters stand by -
Idris Abdul Malik, who reached Mount Arafat after a 24-hour journey from battle-scarred Benghazi, Libya, said "the privilege of visiting this holy land" made the difficulty of his journey meaningless.
"We are very lucky that we will be on Mount Arafat on a Friday... We hope God accepts our prayers.
"Prophet Mohammed has said that Friday is the best day the sun rises on."
The Arab News reported that a recent French convert to Islam had driven 7,000 kilometres (4,340 miles) from North Africa to take part in the hajj.
The movement of pilgrims to Mina on Thursday marked the official start of hajj on the eighth day of the Muslim calendar month of Dhul Hijja.
Security has not noticeably increased around the holy sites, but an AFP reporter observed three checkpoints between Jeddah and Mecca, where security officers verify that visitors hold hajj permits.
Officials say they have intensified efforts to stop people attending hajj without authorisation, as part of safety measures for such a large gathering with massive logistical challenges.
Eighteen aircraft and Black Hawk helicopters will patrol and be on standby for emergencies including "terrorist attacks", Arab News reported.
Supplementing the 85,000 security and civil defence officers who are reportedly deployed for hajj are thousands of health workers.
The medical staff are striving to protect pilgrims from two deadly viruses, Ebola and the MERS coronavirus.
No Ebola cases have yet been found in the desert kingdom.
And "no infectious cases have been recorded among the pilgrims, including coronavirus (MERS)," Acting Health Minister Adel Fakieh said, quoted by official media.