AFP--  Arbaeen is one of the biggest religious festivals on earth and marks the end of the 40-day mourning period for the 7th-century killing of Imam Hussein by the forces of the Caliph Yazid -- a formative event in Shia Islam.

Under tight security, around 14 million worshippers crowded into the golden-domed mausoleum where the Prophet Mohammed's grandson is buried, Karbala's religious authorities said, beating their chests in unison against a background of religious music.

Imam Hussein's killing in the Battle of Karbala in 680 AD was part of a fierce dispute over who should succeed the Prophet, which eventually developed into a bitter schism between the Sunni and Shia branches of Islam.

Tens of thousands of security personnel and Shia militiamen were deployed, as in past years, around the perimeters of the sanctuary as well as on all roads leading to Karbala, about 80 kilometres (50 miles) southwest of Baghdad.

The expulsion of ISIS from Iraqi cities has added an extra dimension to this year's pilgrimage, as the Sunni extremist group has repeatedly targeted Shias. Last year, a suicide bombing killed at least 70 mainly Iranian worshippers returning from the commemoration.

The terrorists have seen their self-styled "caliphate" disintegrate on the battlefield, and are currently fighting to hold a last pocket of territory in Iraq on the Syrian border.

Iraqi Prime Minister Haider al-Abadi said the security operation to protect the millions of pilgrims was a "success" and "another victory" for Iraqi forces who have routed ISIS from most of the country.