Millions of Muslims have gathered in Mecca, marking the beginning of the five-day Hajj pilgrimage—the world's largest annual religious gathering. It is year 1446 in the Islamic calendar. Pilgrims are expected to retrace the steps of the Prophet Muhammad,

Here is a look at the hajj, one of the largest mass gatherings in the world, which starts in Saudi Arabia on Wednesday.

Tens of thousands of Muslim pilgrims clambered up rocks and nestled between boulders on Thursday as they embarked on a day of worship under a scorching sun at the Hajj in Saudi Arabia.