Pilgrims come to be absolved of their sins
The event, which takes them in the footsteps of Mohammed to the barren plain of Mina and the slopes of Mount Arafat, is the biggest yearly mass movement of people on the planet.
This year's Hajj has been overshadowed by the fallout from last September's attacks on the United States and the war in Afghanistan.
But the passion of the believers, for whom the pilgrimage is meant to cleanse the soul, was as fervent as ever as they drove in their hundreds of thousands out of the city.
There were mile-long traffic jams as buses left Mecca, some with pilgrims riding on top after a day spent waiting in the scorching Saudi sun.
"At thy service, my God, at thy service," they chanted in clouds of dust and smog, heading for the city of white tents pitched in the desert.
Obligation
For all the efforts of Muslim leaders to distance themselves from 11 September, many in the West still associate the violence - however nebulously - with Islam, as the BBC's religious affairs correspondent Mark Duff reports.
But the journey is a once-in-a-lifetime obligation for every physically-able Muslim who can afford it and about 1.35m of the pilgrims are from outside Saudi Arabia.
They include about 200,000 people from the world's biggest Muslim nation, Indonesia, and about 10,000 American Muslims - an increase on last year despite earlier fears that many Americans would be deterred from going.
Arriving pilgrims faced stringent security checks, including digital eye tests, as they entered Saudi Arabia on their way to Mecca.
One Egyptian pilgrim, Hasan Ahmed, acknowledged the damage done to his religion by the suicide hijackers who were mainly Saudi by nationality.
"A few crazy men give all of us Muslims such a bad name," he said.
"It will take years for everyone to forget 11 September and the identities of the perpetrators."
But there was militant feeling among some of the pilgrims, too, with one, Ahmed Magbour from Egypt, calling for the "crushing" of Islam's enemies, the Associated Press news agency reports.
Security concerns
The concentration of so many people in a relatively small area has in the past proved a recipe for disaster.
Fire and stampedes have claimed the lives of hundreds of people over the years.
But it is the potential for political unrest that is of greatest concern to the Saudis this year.
In 1987 some 400 people died during clashes between security forces and Iranian pilgrims protesting against Israel and the United States.
This year, a message issued by the Iranian spiritual leader, Ayatollah Khamenei, to the pilgrims accuses Washington of using 11 September as a "pretext for coercive policies".
The Saudi authorities have insisted that they will not tolerate any attempt to exploit the Hajj for political purposes.
One senior Saudi security official boasted that security was so good this year that police and guards inside Mecca were unarmed.
"In other places of the world police are heavily armed even in sports events," said Brigadier Mansour Turki.