Hundreds of thousands of Muslims began Sunday the annual pilgrimage of hajj, pouring from the holy city of Mecca to the tent city of Mina.

This year's pilgrimage season is estimated to be attended by up to 2.5 million pilgrims.

The trip to Mina marks the launch of the annual season of hajj, one of the five pillars of Islam.

Islam stipulates that every able-bodied adult follower who can financially afford the trip must perform hajj at least once in a lifetime.

Pilgrims would spend the Sunday night, marking the eighth day of Dhul Hijja month, in the Mina tent city before heading Monday morning to Mount Arafat about 10 kilometers to the south east.

At Mount Arafat, the pilgrims would spend the day hours in prayer and reading Quran. After sunset, the pilgrims will head to their next stop at al-Muzdalifah, halfway between Mount Arafat and Mina, where they will spend the night and collect pebbles for Ramy al-Jamarat (stoning of the devil) ritual.

On Tuesday, which will mark the first day of Eidul-Adha, the pilgrims will return to Mina after al-Fajr (dawn) prayers and perform the first stage of Ramy al-Jamarat, which they continue performing over the remaining three days of the pilgrimage.

They will then head to Mecca for taking the symbolic march around the Kaaba, the last leg of the hajj.

Saudi-owned Al Arabiya news channel reported that a small fire has broken out in a sector of tents in Mina occupied by Egyptian pilgrims, with no casualties reported.

Hajj has been marred in the past by fires, deadly stampedes and hotel collapses.

On Saturday, Saudi Arabia launched the China-built Mecca Metro, the first dual-track light railway in the kingdom, with the aim of easing the pilgrim traffic during the hajj.

The 18-km light railway, with its initial number of nine stations, links between Mecca and the holy sites of Mina, Mount Arafat and al-Muzdalifah and is designed to carry up to 72,000 passengers an hour.

The 1.8-billion-U.S.-dollar railway is the first cooperation project between China and Saudi Arabia after the two countries singed an agreement in June 2008 to enhance cooperation on infrastructure construction. It is also the first light rail system built by Chinese companies in the Middle East.