With 50,000 security personnel in place, more than two million Muslim pilgrims, wearing white robes, arrived in and around Mecca to participate in the five-day annual religious ritual known as the hajj.
The central rite of the hajj occurred Wednesday with pilgrims performing prayers for forgiveness from Allah at Mount Arafat, where the Prophet Mohammed is said to have delivered his last sermon, some 14 centuries ago.
For the pilgrims, praying at Mount Arafat is a key part of the hajj because it represents the Day of Judgement, and it is believed the prayers will wipe away accumulated sins.
The Saudi kingdom, expressing concerns about terrorism, has deployed a record number of security forces in and around the holiest sites in Islam. There are also concerns about the safety of the huge crowds of pilgrims. In recent years, stampedes and fires have resulted in hundreds of deaths.
Last year, more than 240 people were killed in stampedes. Since then, the kingdom has taken steps to modernize areas where pilgrims pray and perform Islamic rituals. Many areas have been widened in an effort to increase the capacity and security of the crowds.
Thursday, pilgrims will throw stones at three pillars that symbolize the devil, in a ritual that is seen as rejecting temptation. Taller and wider pillars have been erected so that more pilgrims can throw stones at the same time.
All able-bodied Muslims who can afford the trip to Mecca must make the pilgrimage at least once during their lives.
According to Saudi officials, more than a million sheep have been imported for slaughter on the day after the hajj to mark the beginning of Eid al-Adha, or the Feast of Sacrifice, which officially begins Thursday.