Arbaeen marks the 40th day after Ashura, the anniversary of the martyrdom of Imam Hussein (AS) and his faithful companions in the 680AD Battle of Karbala.
Each year, a huge crowd of Shias march toward Karbala, where the holy shrine of Imam Hussein (AS) is located, to perform mourning rites.
According to Qaaf news agency, millions of pilgrims have convened in Karbala a day before Arbaeen and millions more are on their way to the holy city.