Amid tight security, huge crowds of mourners have reached the holy city of Karbala to commemorate the martyrdom of Imam Hussein (PBUH), the third Shia Imam.
The event marks the 10th day of the Lunar month of Muharam, which is also the climax of the ten-day Ashura commemorations.
Around 28,000 soldiers and law enforcement agents are on duty in the Iraqi city. Some 7,000 reserves could join the force if deemed necessary.
Iraqi security forces have reportedly rounded up as many as 80 suspected militants, who intended to unleash chaos on the holy city.
Muharram ceremonies in Iraq have been the target of numerous attacks over the past years. The deadliest took place in 2004 when near-simultaneous blasts in Baghdad and Karbala killed more than 170 people.
Mourning ceremonies were also held in different countries around the world on Thursday.
The Day of Ashura is when the third Shia Imam was martyred along with 72 of his companions while fighting for justice during the battle of Karbala in the year 680.
Seen as one of the most important occasions in the Shia Muslim calendar, Ashura is both a day of sorrow and inspiration. Muslim men, women and children take to the streets every year to express their grief.