Saudi Arabia has barred more than 269,000 people without permits for the annual Hajj pilgrimage from entering Mecca this week, authorities said, as the world's largest annual gathering of people has officially begun.

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.

Tens of thousands of pilgrims in Saudi Arabia utilized the Metro service to travel between the holy sites during this year's annual Hajj season, the Saudi Transport Ministry said.