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Millions of Shia Muslims from across the world have flocked to Iraq\'s holy city of Karbala to hold mourning ceremonies marking Arba\'een.

Arba\'een marks the 40th day since the martyrdom anniversary of Imam Hussein, Prophet Mohammed\'s grandson and the third Shia imam. Security is tight in Karbala and other cities across Iraq. Authorities estimate that some 15 million people have braved the cold and deadly violence to attend Arba\'een ceremonies. Dozens of pilgrims have so far lost their lives in multiple terrorist attacks in recent days.