At least 2.5 million Muslims began the annual hajj pilgrimage on Sunday, heading to an encampment near the holy city of Mecca to retrace the route taken by the Prophet Mohammad 14 centuries ago.
Travelling on foot, by public transport and in private cars, the pilgrims will stream through a mountain pass to a valley at Mina, some three km (two miles) outside Mecca. The path is the same as the Prophet himself took on his last pilgrimage.
The hajj, one of the world's biggest displays of mass religious devotion lasts for five days.
The passage to Mina marks the official launch of the hajj on the eighth day of the Muslim calendar month of Dhul Hijja.
The day is known as Tarwiah (Watering) as pilgrims in the past stopped at Mina to water their animals and stock up for the following day's trip to Mount Arafat..
On Monday, the pilgrims move on to Mount Arafat and its surrounding plain, some 10 kilometers (six miles) to the southeast, where they spend the day in prayer and reflection.
After sunset, they move to Muzdalifah, halfway between Mount Arafat and Mina, where they spend the night.
On Tuesday, the first day of Eid Al-Adha or the Feast of the Sacrifice, the pilgrims head back to Mina after dawn prayers.
They then perform the first stage of the symbolic "stoning of the devil" and make the ritual sacrifice of an animal, usually a lamb.
During the remaining three days of the hajj, the pilgrims continue the ritual stoning before performing the circumambulation of the Kaaba shrine in Mecca and heading home.
Quarter of the world's population
Islam is now embraced by a quarter of the world's population and hajj is a duty for all able-bodied Muslims who can afford it. Many wait for years to get a visa.
Authorities say permits have been granted to 1.7 million foreign pilgrims, with a further 200,000 or so issued to pilgrims from within Saudi Arabia and from neighboring Gulf states.
This year has seen a crackdown on pilgrims who do not have the requisite papers as authorities attempt to prevent numbers getting out of hand.
"I can't explain the feeling of being here," said Mahboob Bangosh, a Canadian pilgrim from Toronto of Afghani origin.
To minimize the risk of overcrowding and to lessen congestion on the roads the authorities will for the first time be operating a Chinese-built train that will call at hajj sites.
The $1.8 billion railway project has tracks that are 18 kilometers long and will transport 180,000 passengers this year, said Habib Zein Al Abideen, assistant minister for municipal and rural affairs.
"We will have a capacity of 72,000 passengers per hour next year. This year we operate at 35 percent capacity. Next year we could have 500,000 to 600,000 passengers," Abideen said.
Due to its limited capacity, the train will this year only carry residents of Saudi Arabia or other Gulf Arabs and next year will open to others nationalities, he said.
It will be big improvement. Tickets cost only a symbolic amount," said Walid al-Mushawer, a Saudi pilgrim.
Saudi Arabia has worked hard to improve facilities to ease the flow of pilgrims at hajj.