At least 31 people were killed and about 100 were injured when a walkway collapsed and set off a stampede in the Iraqi city of Karbala on Tuesday as thousands of Shiite Muslims marked one of the most solemn holy days of the year, officials said.

It was the deadliest stampede in recent history during Ashura commemorations, when hundreds of thousands of people converge on the city, some 80 kilometers (50 miles) south of Baghdad, for the occasion every year.

The incident happened toward the end of the Ashura procession, causing a panicked rush among worshippers near the gold-domed Imam Hussein shrine, according to two officials who spoke to The Associated Press from Karbala.

Tuesday’s commemorations were peaceful until the walkway collapsed, triggering the chaos.

The incident took place during the so-called “Tweireej” run, when tens of thousands of people run toward the shrine of Imam Hussein in Karbala around noon.