Police and troops were out in force as hundreds of thousands of pilgrims massed in the Iraqi shrine city of Karbala to commemorate Ashura.
Tens of thousands more rallied in Beirut, where the head of the Shiite militant Hezbollah movement, Hassan Nasrallah, pledged "victory" against the Sunni extremists of IS.
This year's marking of the day has taken on new meaning after IS seized control of large parts of Iraq and Syria.
The jihadists consider Shiites heretics and have targeted them in deadly attacks, a string of which killed more than 40 people in Baghdad alone in the 48 hours preceding the peak of Ashura today.
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The commemorations mark the killing of Imam Hussein, the grandson of the Prophet Mohammed, by the army of the Caliph Yazid in 680 AD, which helped solidify the divide between what would become the Sunni and Shiite branches of Islam.
A small minority of Shiites mark the day with a self-flagellation ritual called "tatbeer", cutting their heads with swords and spears in mourning for the imam.
Thousands dressed in black took part in a ritual run to the shrine of Imam Hussein, before men on horseback re-enacted an attack on his camp, setting fire to a tent that collapsed in a sheet of flame, sending a dark cloud of smoke rising over a massive crowd.
There were no reports of attacks against the pilgrims in Iraq today, after more than 25,000 members of the security forces were deployed in Karbala itself and thousands more in Baghdad and along routes to the city.
"These takfiris will be defeated in all areas and countries, and we will feel honoured that we played a role in their defeat," he said by video link.
Hezbollah has sent thousands of fighters into neighbouring Syria to support the troops of President Bashar al-Assad against the mainly Sunni rebels battling his regime.