Shi'ite pilgrims beat their chests in ritual mourning for Imam Hussein, the Prophet Mohammad's grandson, at Imam Abbas shrine in Karbala REUTERS/Atef Hassan

20 million Shia Muslims - as well as some Sunnis, Christians, Yazidi and other faiths - are on their way to Karbala, Iraq to participate in the world's largest annual gathering of people, the religious pilgrimage of Arbaeen.

The pilgrims must reach Karbala, south of Baghdad, by 12th December, a date which marks the end of 40 days' fasting following Ashura, the ritual which marks the death of Mohammad's grandson Imam Hussein. Karbala is the site of a shrine in Hussein's name on the spot where Hussein and his army were killed and beheaded by the army of Caliph Yazid in 680.

Many Shia walk from Najaf, a distance of 55 miles, and some even make the 425-mile journey from the port of Basra on foot, which takes two weeks, passing through hostile territory, and deserts which are baking during the day and freezing at night. The pilgrimage was banned under Saddam Hussein and only reintroduced in 2003.

Isis view Shia Muslims as aspotates Reuters

Many Sunni Muslims, particularly extremists such as Isis (known in the local language as Daesh) regard Shia Muslims as apostates and attack both believers and their places of worship. The Arbaeen pilgrimage itself has often been targeted by Isis and other fanatical Sunnis. Last year Sunni militants attacked the pilgrimage with suicide bombers and rockets, leaving dozens dead.

Although Isis have become notorious for carrying out sectarian slayings, some Shia militias have also killed many Sunnis. Amnesty International says it has evidence that many Shia death squads are backed by the Shia-led government.

Despite the ongoing sectarian violence 20 million people now attend Arbaeen, making it the world's largest annual gathering in one place. Although the Hindu Kumbh Mela is believed to be even larger it is only held every three years. The pilgrimage of Hajj, to which all adult Muslims - Sunni and Shia - are expected to attend at least once in their lifetime if possible, draws around two million each year.