The expulsion of Daesh from Iraqi cities added an extra dimension to this year's pilgrimage.
Iraq says more than 13 million people have gathered in the city of Karbala to mark Arbaeen, the annual commemoration marking the end of the mourning period for the martyrdom of Hussein, the grandson of Prophet Muhammad.
The expulsion of Daesh from Iraqi cities added an extra dimension to this year's pilgrimage to Hussein's shrine in Karbala, about 80 km southwest of Baghdad.
Iran's spiritual leader Ali al Sistani praised Iraqi security forces Friday for securing the routes in and out of Karbala for visitors travelling from across Iraq and beyond.
Hussein, along with his companions, was killed at the Battle of Karbala in 680 AD by the forces of Yazid I of the Umayyad caliph.
The holiday is observed by Shia communities worldwide, with large gatherings in Iran and southern Lebanon.