Huge crowds of black-clad Shiite Muslim pilgrims thronged the Iraqi shrine city of Karbala on Wednesday, weeping and beating their chests in mourning for the seventh century killing of the prophet's grandson.



Shiites in Iraq have come under frequent attack by Sunni extremists of ISIS who regard them as heretics, and some 30,000 security personnel were out on the streets to guard the pilgrims, although attacks in Karbala are rare.



The annual Ashoura commemorations, which peak on Wednesday, mark the killing of Imam Hussein by the forces of the Caliph Yazid in 680 AD -- a formative event in Shiite Islam.



The pilgrimage draws huge numbers of faithful, with Karbala deputy governor Ali al-Mayali saying three million were expected to take part this year.



Imam Hussein's death was part of a dispute over who should succeed the Prophet Mohammed, which eventually developed into a bitter schism between the Sunni and Shiite branches of Islam.



Some Muslims, who became known as Shiites, believed that a blood relative of the Prophet Mohammed should succeed him as the spiritual and temporal leader of Muslims, and backed his cousin and son-in-law Ali -- Hussein's father -- as successor.



Others, now known as Sunnis, insisted that relationship to the prophet by blood was not required -- a position that carried the day for his three immediate successors before Imam Ali became the fourth.



Muawiyah, who founded the Umayyad dynasty, took power as caliph on Ali's death, and, according to Shiite tradition, named his son as successor in violation of an agreement under which Imam Hussein should have succeeded.



According to Shiite belief, Imam Hussein went knowingly to his death at the hands of Yazid's forces in what is now Iraq in a bid to expose the corruption and irreligiosity of his rule.



This ideal of self-sacrifice is a key tenet of Shiite Islam to this day, inspiring followers to give their lives for causes, including the war against ISIS.



Iraqi forces are currently preparing for a final push on Mosul, the last city in Iraq held by ISIS, which has lost much of the ground it seized in 2014.