Millions of Muslims have flocked to Mecca to celebrate Islam's biggest holiday - Eid-al-Adha, the festival of sacrifice.

Pilgrims performing the annual Hajj in the holy city on Tuesday, cast pebbles at three stone walls representing Satan in a symbolic rejection of temptation, with nearly three million people set to file past the walls and pelt them with stones.

As a result, the stoning ritual is often thought of as the most hazardous of the pilgrimage. Crushes and pileups have killed hundreds, most recently in 2006.

It appeared that a greater number than usual of "unofficial" pilgrims were performing the hajj this year, sneaking in without going through tour groups as required by Saudi authorities and sleep along roadsides in tents during the four days of rituals.

The first day of stoning also marks the start of the Islamic holiday of Eid al-Adha, or feast of sacrifice, when Muslims around the world slaughter sheep and cattle in remembrance of Abraham's near-sacrifice of his son.

After performing the stoning ritual, pilgrims bought tokens to have a sheep slaughtered and the meat distributed to the poor.