The five-day hajj pilgrimage represents one of the world's biggest gatherings every year, and is required of all able-bodied Muslims once in their life.
"It's the dream of every Muslim to come here to Mecca," Frenchman Soliman Ben Mohri said.
"It's the ultimate journey. What worries me is the return to my normal life. For the moment, I am in a dream."
The hajj offers pilgrims an opportunity to feel closer to God amid the Muslim world's many challenges, including the threat of violence in the Mideast and the plight of Myanmar's Muslim Rohingya minority.
"We are very blessed by Allah to be in this place, and we pray to Allah to make the Islamic nations from the West to the East in a better situation," said Essam-Eddin Afifi, a pilgrim from Egypt. "We pray for the Islamic nations to overcome their enemies."
Muslims believe the hajj retraces the footsteps of the Prophet Muhammad, as well as those of the prophets Ibrahim and Ismail — Abraham and Ishmael in the Bible. Muslims believe God stayed the hand of Ibrahim after commanding him to sacrifice his son, Ismail. In the Christian and Jewish version of the story, Abraham is ordered to kill his other son, Isaac.
The Kaaba represents the metaphorical house of God and the oneness of God.
Muslims circle the Kaaba counter-clockwise seven times while reciting supplications to God, then walk between the two hills traveled by Hagar, Ibrahim's wife. Mecca's Grand Mosque, the world's largest, encompasses the Kaaba and the two hills.
Before heading to Mecca, many pilgrims visit the city of Medina, where the Prophet Muhammad is buried and where he built his first mosque.
After prayers in Mecca, pilgrims will head to an area called Mount Arafat on Monday, where the Prophet Muhammad delivered his final sermon. From there, pilgrims will head to an area called Muzdalifa, picking up pebbles along the way for a symbolic stoning of the devil and a casting away of sins that takes place in the Mina valley for three days.
At the hajj's end, male pilgrims will shave their hair and women will cut a lock of hair in a sign of renewal for completing the pilgrimage. Around the world, Muslims will mark the end of hajj with a celebration called Eid al-Adha. The holiday, remembering Ibrahim's willingness to sacrifice his son, sees Muslims slaughter sheep and cattle, distributing the meat to the poor.
Maj. Gen. Mansour al-Turki, the spokesman of the Saudi Interior Ministry, told journalists Saturday that over 2 million Muslims from abroad and inside the kingdom would be taking part in this year's hajj.
Saudi Arabia's ruling Al Saud family stakes its legitimacy in part on its management of the holiest sites in Islam. King Salman's official title is the "Custodian of the Two Holy Mosques," at Mecca and Medina. Other Saudi kings, and the Ottoman rulers of the Hijaz region before them, all have adopted the honorary title.
The kingdom has spent billions of dollars of its vast oil revenues on security and safety measures, particularly in Mina, where some of the hajj's deadliest incidents have occurred.
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.
This year, the Saudis have launched a "smart hajj" initiative, with apps to help pilgrims with everything from travel plans to medical care.
