FACTBOX: Facts about the annual haj or Muslim pilgrimage

MECCA, Saudi Arabia (Reuters) - The annual haj pilgrimage starts in the holy city of Mecca on Monday. Here are five facts about the event, the largest regular religious gathering in the world.

-- The haj is one of the five basic obligations of a Muslim, along with a short creed, prayer, giving alms, and fasting during the month of Ramadan. Every Muslim who has the means should perform the pilgrimage at least once in his lifetime.

-- The haj is older than Islam and took place annually during the childhood of the Prophet Mohammad when he grew up in Mecca in the late 6th century. Muslims trace its origins to an ancient monotheistic cult associated with the prophet Ibrahim, known to Jews and Christians as the patriarch Abraham.

-- The pilgrimage itself takes at least six days, starting on the 8th day of the lunar month of Dhul Hijja, which falls this year on Monday. But most pilgrims come earlier and first perform the rites of the umra, or lesser pilgrimage, which are confined to central Mecca.

-- The haj itinerary takes pilgrims out of Mecca to the plain of Arafat about 15 km (10 miles) to the east, where they spend the afternoon of the second day. They then return to Mecca in stages, stopping on the way to throw pebbles at a wall, representing defiance of the devil. In Mecca they repeat the rites of the umra, including the circumambulation of the Kaaba, the cube-shaped stone shrine at the centre of the Grand Mosque.

-- This year 1.6 million pilgrims have entered Saudi Arabia for the pilgrimage. Pilgrims from within Saudi Arabia, either Saudis or foreign residents, will bring the total well beyond 2 million.

(Writing by Jonathan Wright; Editing by Michael Winfrey)