Shia Muslims in Iran have taken to the streets and religious centers across the country to honor the 40th day after the martyrdom anniversary of Imam Hussein, the third Shia Imam.

Arbaeen is the 40th day after Ashura, the tenth day of the lunar calendar month of Muharram.

The grandsom of Prophet Muhammad was martyred by the tyrant of the time, Yazid, more than a thousand year ago on Ashura.

Shias hold mourning processions during the first nine days of Muharram in anticipation of Ashura.

Arbaeen is also known as the world’s largest annual pilgrimage.

The occasion has brought millions of Muslims to Iraqi city of Karbala, where Imam Hussein and his companions were martyred.

Millions of Iranians travel to Iraq each year to walk the distance between the Iraqi holy cities of Najaf and Karbala, which houses Imam Hussein’s holy shrine.

The Iranian pilgrims are joined by other Shia Muslims from various countries along the course.

Imam Hussein is highly respected among both Shias and Sunnis.

Some members of Iraqi Christians also join the march to Karbala.