Some 30,000 security personnel were out on the streets to guard the pilgrims, although terrorist attacks in Karbala by Daesh (ISIL) are rare.
The annual Ashura commemorations mark the martyrdom of Imam Hussein (AS) by the forces of the ruler of the time, Yazid, in 680 AD.
The pilgrimage draws huge numbers of faithful, with Karbala deputy governor Ali al-Mayali saying three million were expected to take part this year, AFP reported.
Imam Hussein (AS) went knowingly to his death at the hands of Yazid's forces in what is now Iraq in a bid to expose the corruption and irreligiosity of his rule.
Every year on Tasua and Ashura, the 9th and 10th days of the lunar month of Muharram, falling on Tuesday and Wednesday this year, millions of Shiites around the world, dressed in black, take to the streets and mosques to mourn the martyrdom of Imam Hussein (AS).
The mourning ceremonies in Iraq come as Iraqi forces are currently preparing for a final push on Mosul, the last city in the country held by Daesh, which has lost much of the ground it seized in 2014.