Millions of Muslims around the world are celebrating the annual Eid al-Adha holiday, which coincides with the second day of the Hajj pilgrimage to Mecca, the birthplace of Islam.

Eid al-Adha, or the "Feast of the Sacrifice", is one of the biggest events in the Muslim calendar, marking Abraham's willingness to sacrifice his own son as an act of obedience to God. At the last minute, God intervened, and gave Abraham a ram to sacrifice instead.

Traditionally, Muslims sacrifice an animal at Eid and share the meat with their families, neighbours, and people less fortunate than themselves.

On Saturday, vast crowds of pilgrims dressed in white to symbolise purity and equality under God - marked the beginning of the annual Hajj pilgrimage by climbing Mount Arafat, just outside the Saudi Arabian city of Mecca, where Islam's Prophet Muhammad is said to have delivered his farewell sermon.

The ascent of Arafat is the first event associated with the five-day Hajj, a time to seek forgiveness for one's sins and for individual meditation on the faith.

Hajj consists of a series of detailed rituals in Mina, Muzdalifa, Arafat and Mecca.

Saudi authorities say that an estimated 2.5 million pilgrims are expected to participate.

One of the five pillars of Islam, Hajj is a religious duty that must be carried out at least once by every able-bodied Muslim who can afford it.

This year's pilgrimage is unique as it is the first to happen since the Arab Spring began last year.