(tasnim) -- A main road to southern Tehran leading to the holy shrine of Abdol Azim al-Hasani (AS) in Rey was blocked to motorists on Thursday to allow a packed crowd to go on the pilgrimage on foot.

Since a couple of years ago, Tehran residents, calling themselves those “left behind” (for not having had the opportunity to take part in the annual Arbaeen march in Iraq), walk long distances from different parts of Tehran to Rey on Arbaeen.

Security around the sacred shrine of Abdol Azim al-Hasani (AS) has been tightened for the procession.

Arbaeen is a national holiday in Iran. It marks the 40th day after Ashura, the martyrdom anniversary of Imam Hussein (AS), the grandson of Prophet Mohammad (PBUH), in 680 AD.

Last year, around 20 million pilgrims gathered in Karbala, Iraq, on Arbaeen, making the world’s hugest human gathering in modern history.