The ceremony is underway in the Imam Khomeini Mosque in downtown Tehran.
The Arbaeen mourning ceremony, falling on November 9 this year in Iran, is one of the largest religious gatherings in the world.
It marks the 40th day after the martyrdom anniversary of the grandson of Prophet Mohammad (PBUH), Imam Hussein (AS), the third Imam of Shiite Muslims.
The mourning ceremony began and was soon established when Jabir ibn Abdullah Ansari, a close follower of Prophet Mohammad (PBUH), visited Imam Hussein's grave as the Imam first pilgrim, 40 days after Ashura.
