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Iraq's top Shiite cleric says more than 13 million have gathered in the holy city of Karbala to mark Arbaeen, the annual commemoration marking the end of the mourning period for the martyrdom of Imam Hussein, the Prophet Muhammad's grandson and a central figure in Shiite Islam.

Grand Ayatollah Ali al-Sistani praised Iraqi security forces Friday for securing the routes in and out of Karbala for visitors traveling from across Iraq and beyond.

Sheikh Abdul-Mahdi al-Karbalai, Sistani's representative, delivered the cleric's message following Friday prayers.

Imam Hussein was killed in a climactic battle in Karbala in A.D. 680 that cemented the rift between Sunnis and Shiites.

The Arbaeen holiday is observed in Shiite communities worldwide, with large gatherings in Iran and southern Lebanon.