Two million people are expected to take part in this year's pilgrimage
Hundreds of thousands of pilgrims from around the world on Tuesday began moving from the holy city of Mecca to nearby Mina for the start of the Hajj, the world's largest annual gathering.
Around two million people are expected to take part in this year's pilgrimage.
"It is a gift from God that He has chosen us to come here," said Walaa Ali, a 35-year-old Egyptian pilgrim with tears in her eyes. "I am so happy to be here."
Nearby, both men and women sat side by side listening to preachers explain the history and rituals of Hajj.
The first day of Hajj is known as Tarwiah Day, when pilgrims traditionally watered their animals and stocked water for their trip to Mount Arafat, about 10 kilometres (six miles) southeast of Mina.
Nowadays pilgrims spend their time there in prayer and reciting the Koran.
The climax of the Hajj season is on Arafat Day, which falls on Wednesday.
With the start of Hajj, pilgrims enter the stage of ihram -- a state of purity in which they must not wear perfume, cut their nails, or trim their hair or beards.
During ihram, men wear a seamless two-piece shroud-like white garment, symbolising resurrection and emphasising unity regardless of social status or nationality.
Women must wear loose dresses exposing only their faces and hands.
They are following the 1,400-year-old tradition of the Prophet Mohammed.
The Hajj is among the five pillars of Islam and every capable Muslim must perform the pilgrimage at least once in their life.
It had been largely incident-free for the past decade amid an increase in safety improvements.
But on 11 September, during severe winds, a construction crane toppled into a courtyard of Mecca's Grand Mosque, killing 109 people and injuring nearly 400.
Saudis, Iranians, Nigerians, Malaysians, Indonesians and Indians were among the dead.
This year's challenges
Authorities say they are on the alert for possible attacks by the Islamic State (IS) militant group, which has carried out bombings targeting mosques in the kingdom in recent months.
Security forces have taken "measures to prevent terrorist groups from exploiting Hajj season to carry out acts of sabotage," said interior ministry spokesman General Mansur al-Turki.
The ministry says 100,000 police have been deployed to secure Hajj.
"We take all possibilities into consideration during Hajj. This includes the kingdom being targeted by terrorist organisations," Turki told AFP.
Saudi Arabia is also at war this year, leading an Arab coalition conducting air strikes and supporting local and government forces in Yemen against Houthi militia since March.
Most Yemeni pilgrims performing Hajj this year are already residing in the kingdom.
Among other challenges facing Saudi authorities this year is potential transmission of the deadly Middle East Respiratory Syndrome coronavirus (MERS-CoV).
The capital Riyadh saw a jump in infections last month.
But health officials have insisted that so far no MERS infections have been recorded among pilgrims.
Saudi Arabia is the country worst affected by MERS, with 528 deaths since the virus appeared in 2012.
The health ministry has mobilised thousands of health workers to help secure a virus-free pilgrimage.