At mosques and shrines across Iraq, millions of Shi'ites, Iraqis and foreigners, commemorated the slaying of Prophet Mohammad's grandson Hussein at the battle of Kerbala in 680AD, an event that defines Shi'ism and its rift with Sunni Islam. Loudspeakers blared out Ashura chants across the city of Kerbala, where hundreds of thousands of pilgrims dressed in black gathered outside the golden-domed Imam Hussein shrine.

Huge crowds of pilgrims, most wearing black shirts and head-dresses packed inside the Imam al-Hussein shrine praying fervently, beating their breasts and wailing for the loss of their martyr, while hundreds of thousands more coursed through the streets outside, waving green flags representing Islam and red flags for the blood of Hussein.

The rituals are designed to express Shi'ites' sense of guilt for not having come to Hussein's aid as he faced down an army of opponents, and allow believers to share in the suffering of the martyr, whose death was a key factor in the conflict between Sunni and Shi'ite Muslims.

Saddam Hussein, a Sunni, placed strict limits on the traditional pilgrimage to Kerbala, but since his overthrow in 2003 Ashura has become a show of strength for Iraq's Shi'ite majority and a prime target of Sunni Islamist insurgents.

Islamic State (IS), seen as more ruthless than al-Qaeda, consider Shi'ites heretics due to their veneration of Hussein and Prophet Muhammad's family, who are respected but not held in the same regard by Sunnis.