Iraq's Health Ministry says the number of those killed in a stampede on a bridge over the River Tigris in Baghdad is expected to reach 1,000.
At least 956 people are confirmed dead with 465 wounded. Most of the victims were women and children.
Today saw the heaviest death toll in Iraq since the US-led invasion in March 2003.
The panic began as about one million pilgrims were making their way to a mosque to celebrate the martyrdom of Musa Al-Kadhim, a revered religious figure among Shi'ites.
A rumour spread that a suicide bomber was among them, and hundreds of people ran onto the bridge.
Many were crushed to death and others fell into the river when the railings collapsed.
The stampede occurred after the Kadhimiya mosque - the burial place of Shi'ite imam Mussa Kazim who died 12 centuries ago - came under mortar fire, leaving at least seven dead and 37 wounded.
An al-Qaeda linked group calling itself the Jaiech Al-Taifa al-Mansoura claimed it carried out the attack to 'punish the genocides committed against Sunnis'.
The Iraqi Prime Minister, Ibrahim Jaafari, has declared three days of mourning for the victims.
The tragedy came amid deep divisions in the country over Iraq's draft constitution. It is opposed by disgruntled Sunnis who hope to defeat the charter in a 15 October referendum.
