World's largest pilgrimage in Iraq's Karbala

The holiday marks the end of the forty day mourning period after the anniversary of the 7th century martyrdom of Imam Hussein, the Prophet Muhammad's grandson (Photo: AP)

Shiite faithful pilgrims gather between, the holy shrine of Imam Abbas and the holy shrine of Imam Hussein, in the background (Photo: AP)

Shiite pilgrims make their way to Karbala for Arbaeen, in Basra, 340 miles (550 kilometers) southeast of Baghdad, they walk from Najaf to Karbala (Photo: AP)

Although mostly Shia Muslims from the world participate, Sunnis Muslims, Christians, Yazidis and Zorastrians also attend (Photo: AP)and serve pilgrims

The moon rises over Shiite pilgrims as they march to Karbala for the Arbaeen ritual in Baghdad (Photo: AP)

22 million turned up for the event which is known to be the single largest annual pilgrimage in the world (Photo: AP)

The pilgrimage continues even as a bombing by ISIS on Monday claimed six lives at the start of the pilgrimage (Photo: AP)

Shiite faithful pilgrims pray at the holy shrine of Imam Abbas for Arbaeen in Karbala, 50 miles (80 kilometers) south of Baghdad, Iraq (Photo: AP)

Iranian Shiite pilgrims mourn during Arbaeen ritual, commemorating the end of the forty day mourning period (Photo: AP)