VIDEO: See-off ceremony of Arbaeen pilgrims in Tehran

The Arbaeen mourning ceremony is one of the largest religious gatherings in the world.

Each year, a huge crowd of Shias flock to Karbala, where the holy shrine of Imam Hussein (PBUH), to perform mourning rites. Arbaeen marks the 40th day after Ashura, the martyrdom anniversary of the grandson of Prophet Mohammad (PBUH), Imam Hussein (PBUH).