Arbaeen marks the culmination of a 40-day mourning period after the anniversary of the martyrdom of Hussein, Prophet Muhammad's grandson and the third Shia Imam.
The ceremonies are taking place in Karbala on Thursday amid heavy troop deployment to protect pilgrims against terrorist attacks.
Many of the black-clad mourners have reached the city on foot to show their devotion to the Shia Imam.
Some countries like Iran marked the occasion by holding mourning rituals on Wednesday. Millions of Iranians have also traveled to Iraq to take part in the Arba'een march and attend mourning ceremonies in Karbala.
In the Iranian capital Tehran, thousands of those who could not make it to the Iraqi city, have participated in a symbolic march to honor the memory of the 3rd Shia Imam.
Imam Hussein and 72 of his loyal companions, including his family members, were martyred on Ashura, the tenth day of the lunar month of Muharram, in the battle of Karbala against the second Umayyad caliph, Yazid I, in 680 A.D. Imam Hussein was martyred after he refused to pledge allegiance to the tyrant ruler.
The number of Arba’een pilgrims reportedly hit 20 million in the last two years. Iraqi officials expect the turnout for this year’s Arba'een procession to be higher than the past.
According to Imam Khomeini, the founder of the Islamic Republic, the holy months of Muharram and Safar have kept Islam alive and advised preachers to promote genuine teachings of Islam during this period.
Muharram is the month in which justice has raised up against cruelty while truth has confronted falsehood, and has proved that throughout history truth has always been triumphant against falsehood.
Researchers say the founder of the Islamic Republic believed that the divine events and sacrifices rendered by Imam Hussein on the day of Ashura could bolster among all Muslim communities.