The occasion, known as the world’s largest annual Muslim pilgrimage, comes 40 days after Ashura, the 10th day of the lunar calendar month of Muharram when Imam Hussein, the grandson of Prophet Muhammad (PBUH), was martyred in the Battle of Karbala at the hands of the tyrant of the time, Yazid I, in the seventh century
Each year, Shia and Sunni pilgrims as well as those from other faiths gather in Iraq in the weeks leading to the day of Arbaeen and stream towards Karbala on foot from other Iraqi cities, especially Najaf and Basra.
During the long walks, the pilgrims sleep and eat in tents erected by charity groups and mosques alongside roads.
This year's pilgrimage culminates on Tuesday. Millions of pilgrims have already gathered in Karbala to join the mourning procession around the golden-domed shrines of Imam Hussein and his half-brother Hazrat Abbas.
In recent years, Karbala has been host to between 10 and 20 million visitors during the event.
At least two million pilgrims have passed border checkpoints into Iraq from neighboring Iran.
In the Iranian capital Tehran, thousands of people are taking part in a large symbolic march marking Arbaeen.
Arbaeen is viewed as an overwhelmingly powerful display of Shia solidarity and conveys a message of unity among Muslims against the global arrogance.
In a Twitter post on Monday, Iran’s Foreign Minister Mohammad Javad Zarif said that the annual pilgrimage bears the message that “no tyrant will ever end the struggle for justice.”