Every year, at the start of the last month - called Zil-Hijjah in Arabic - of the Muslim calendar, almost 2.5 million people journey to the east coast of Saudi Arabia for the hadj (al-Hajj in Arabic), the holy pilgrimage to Mecca which all devout Muslims should, if they can, perform once in their lifetime. This host of visitors comes from every corner of the globe where Islam is practised, an astonishing mixture of nationalities.
The city of Mecca was the birthplace of the prophet Muhammad, founder of Islam, in AD 570, and it was he who decreed it should a site of Muslim pilgrimage. The focus of the pilgrimage is the sanctuary of the Kaaba, a cube-shaped building in Mecca which, according to Muslim tradition, was built by Abraham. (It is towards the Kaaba that all Muslims, no matter where in the world they are, turn to pray five times - Fajr, Zohr, Asr, Mughrib and Esha - every day.) Especially holy is the sacred black stone - Hajr-e-Aswad - set into one wall of the building by Muhammad. As the first and last acts of their pilgrimage, all Muslims visit the Kaaba and walk around it seven times; those who can get close enough to the black stone kiss or touch it as they pass by, but because of the crowds most have to be content with waving in its direction. The principal days of ceremony, prayer and meditation are those between the 7th and 10th of the month - and for much of this period, all the pilgrims must be in the same place at the same time.
The annual hadj is an extraordinary feat of organisation. The sheer numbers of people involved pose enormous problems of health, transport and policing. A vast air-conditioned tent city is erected to house the visitors, who outnumber the ordinary inhabitants of Mecca by three to one.
Yet despite the problems, the pilgrimage is becoming ever more popular. The pilgrim (Hadji) returns to his own country with new honour, having fulfilled one of the most sacred obligations of his faith.