Ashoura is observed on the tenth day of Muharram, the first month of the Islamic lunar calendar, by all Muslims.
A walkway collapsed and set off a mass stampede in the Iraqi city of Karbala as thousands of Shia Muslims marked one of the most solemn holy days of the year.
At least 31 people were killed and about 100 were injured, officials said on Tuesday. At least 10 among the wounded pilgrims were in a critical condition.
It was the deadliest stampede in recent history during Ashoura commemorations, when hundreds of thousands of people converge on the city, about 80km south of Baghdad, for the occasion every year.
The deadly rush began when part of a walkway collapsed during a procession, causing mass panic among worshippers.
Shia pilgrims converge on Karbala annually to commemorate the death of Imam Hussein, Prophet Mohammad’s grandson.
He was killed in the year 680 in what would become Karbala by the forces of the Caliph Yazid in a major event that helped solidify the divide between what would become Islam’s Sunni and Shia branches.
People in Iran, Pakistan, Afghanistan, India, Bangladesh, and Lebanon also marked the day with rallies, prayers and self-flagellation.
“Today is arguably the most important day for Shias here in Iraq,” said Al Jazeera’s Charles Stratford, reporting from Karbala.
“We spoke to members of the holy shrine authority earlier and they said more than three million visitors were in the city today for this event. Basically people fell over as they went round the tomb of Hussein, and the sheer number of people has resulted in many people being crushed.”
On Tuesday, packed processions of black-clad worshippers made their way to 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. Others – even young boys – cut incisions into their foreheads with scalpels or large sabres, leaving streams of blood cascading down their faces.
Similar ceremonies took place in the capital Baghdad and in the Iraqi cities of Najaf and Basra.
Officials said the stampede took place during the so-called “Tweireej” run, when tens of thousands of people rush towards the shrine around noon.
The 3km run symbolises when the maternal cousins of Imam Hussein’s half-brother al-Abbas ran from the nearby village of Tweireej to rescue him, only to find out he had been killed.
In recent years, Ashoura processions have been attacked. In 2004, at the height of Iraq’s sectarian violence, 143 people were killed in near simultaneous suicide and other bombings at shrines in Baghdad and Karbala during the Ashoura procession.
Under ex-dictator Saddam Hussein’s Sunni-dominated regime, the vast majority of Ashoura commemorations were banned.
The day is now a national holiday with streets across the country shuttered to allow for elaborate re-enactments of the Battle of Karbala.
This year’s sombre commemoration comes amid rising tensions in the Middle East and a crisis between Iran and the US in the wake of the collapsing nuclear deal between Tehran and world powers.