Shi'ite Muslim pilgrims gathered at the shrine of Imam Hussein in Iraq's holy city of Karbala on Tuesday to mark the annual Arbaeen commemoration.

Dressed in black, they beat their heads and chests as they thronged the gold-domed mausoleum where the grandson of Prophet Muhammad is buried.

Arbaeen marks the 40th day after Ashura, which commemorates the death of Imam Hussein in AD680.

The authorities estimated that over 13 million people made the pilgrimage to the city, located 100km south-west of Baghdad, from around the world, including Lebanon, Pakistan and the United States, reported Agence France-Presse.

Most pilgrims from Iran walked all or part of the way to the shrine. Some even stopped in the western Iranian city of Mehran for days or weeks to provide free services to fellow pilgrims, such as sewing torn clothes and polishing shoes.