More than two million Muslims have begun heading from Mecca to the valley of Mina in Saudi Arabia as part of the annual Hajj pilgrimage amid heavy security.
Draped in white robes, the pilgrims walked or boarded buses to begin tracing the journey that Muslims believe was made by the Prophet Muhammad more than 1,400 years ago.
Saudi authorities estimate that nearly three million pilgrims are attending this year's Hajj, more than 1.6 million from abroad.
Mansur al-Turki, Saudi interior ministry spokesman, said on Thursday: "Everything is proceeding according to plan".
Al-Turki said the final number of pilgrims would not be known until the Hajj climaxes on Friday, when the faithful start moving towards Mount Arafat where they will spend the day praying and asking for God's forgiveness at the summit, symbolic of waiting for the last judgment.
Forces deployed
Thousands of Saudi security forces have been deployed along the routes to be used by the pilgrims.
Official media said security and health authorities have been mobilised to ensure the safety of the faithful during often risky rituals.
For pilgrims streaming in from all continents, the Hajj is a crowning moment of faith, a duty for all able-bodied Muslims to carry out at least once if they can afford to. It is one of the five pillars of Islam.