BAGHDAD: Millions of pilgrims gathered in Iraq’s holy city of Karbala amid tight security Wednesday to mark Arbaeen, the annual commemoration of the end of the 40-day mourning period over the killing of Imam Hussein, the Prophet Mohammad’s grandson, in A.D. 680.

Many made the pilgrimage on foot from the Iraqi capital, walking along roads lined with blast walls and razor-wire, and waiting at checkpoints to be frisked by security forces. Soldiers and police also mingled with the pilgrims, most of whom wore black out of mourning.

Iraqi officials said security was increased this year, with operations coordinated between the Interior Ministry, Shiite militias and Iranian advisers. Daesh (ISIS) has repeatedly targeted Iraq’s Shiite majority.

Iraqi state-run media said Wednesday that more than 22 million pilgrims have visited Karbala, setting a record for the annual pilgrimage. That number could not be independently verified. Iraqi authorities said last year’s Arbaeen drew 17 million pilgrims.

The faithful traditionally walk for a day or longer to reach Karbala where crowds chant and march around the Imam Hussein shrine. The holiday is observed in Shiite communities worldwide, with large gatherings in Iran and southern towns in Lebanon.

On the walk to Karbala, which can take up to two days, pilgrims said security was the main topic of conversation, closely followed by current politics.

Daesh controls large parts of northern and western Iraq, including the city of Ramadi, some 140 kilometers north of Karbala. A suicide bomber targeted pilgrims in Baghdad Monday, killing nine people and wounding another 21, according to Iraqi security officials.

A special task force of Shiite militiamen has been deployed to Karbala to provide extra security, said Hashim al-Musawi, a spokesman for the umbrella group of militias known as the Popular Mobilization Forces. He added that Iranian advisers were also on hand to coordinate security.