At least 31 pilgrims died on Tuesday in a stampede during a commemoration at a major shrine in the Iraqi city of Karbala as they marked the sacred day of Ashura, the health ministry said.
The crush resulted in 75 people also being injured at the shrine around 100 kilometres (60 miles) south of Baghdad, said spokesman Saif al-Badr, stressing that the toll was not final.
It is the deadliest stampede in recent history during Ashura, when Shia Muslim pilgrims from around the world visit Karbala to commemorate the death of Hussein, Prophet Mohammed's grandson.
He was killed in the year 680 by the forces of the Caliph Yazid, a major event which helped solidify the divide between what would become Islam's Sunni and Shia branches.
On Tuesday, packed processions of black-clad worshippers made their way to Imam Hussein's gold-domed shrine in Karbala, carrying black flags with his name written in red and wailing loudly.
Some whipped their backs and chests to demonstrate their sorrow.
Similar ceremonies took place in the capital Baghdad and in the southern cities of Najaf and Basra.
Under former dictator Saddam Hussein's regime, which fell after the US-led invasion of Iraq in 2003, the vast majority of Ashura commemorations were banned.
Now, the day is a national holiday, with streets across the country shuttered to allow for elaborate re-enactments of the Battle of Karbala.
In 2005, at least 965 pilgrims heading to the Imam Kadhim shrine in Baghdad during a different holiday died after rumours of a suicide bomber in the crowd sparked a mass stampede.