Almost 1,000 people are known to have died in a stampede of Shia pilgrims in northern Baghdad, Iraqi health officials have said.
So far, there have been at least 965 confirmed deaths, making the incident the single biggest loss of Iraqi life since the US-led invasion in 2003.
The incident happened on a river bridge as about a million Shias marched to a shrine for a religious festival.
Witnesses said panic spread over rumours of suicide bombers.
Radical Sunni groups have often targeted Shias in the past, but Iraqi officials said the tragedy had nothing to do with sectarian tension.
Many victims, mostly women, children or elderly, were crushed or drowned.
Littered with shoes
The BBC's Jon Brain in Baghdad says that on Wednesday evening the corridors of the city's hospitals were lined with bodies, as were the pavements outside.
MUSLIM FESTIVAL TRAGEDIES
July 1987 - Saudi security forces clash with Iranian pilgrims in the holy city of Mecca, 402 killed
July 90 - 1,426 pilgrims killed in a stampede in Mecca
May 94 - 270 pilgrims killed in Mecca stampede
April 97 - More than 340 pilgrims killed and 1,500 injured in fires at tent city in Mina, Saudi Arabia
Feb 2004 - 251 pilgrims killed in stampede at Mina
Grief-stricken relatives have been walking along the lines, lifting the covers looking for loved ones killed in the tragedy.
The bridge, which spans the River Tigris, is littered with the shoes of victims. According to tradition, they were supposed to walk across barefoot.
Earlier, mortar rounds had been fired into the crowd, killing at least seven people.
About 36 others were injured when four mortar rounds landed close to the Kadhimiya mosque.
A Sunni group, Jaysh al-Taifa al-Mansoura (Army of the Victorious Sect), said it carried out the mortar attacks, according to a statement posted on a website frequently used by groups linked to al-Qaeda.
There were also reports that some worshippers had been poisoned.
Our correspondent says there are now fears that the tragedy will increase the sectarian divisions in this already troubled country.
Iraqis are preparing to vote on a proposed constitution, with Shia and Sunnis sharply divided on its contents.
Wednesday is the last day the majority of Iraqis can register to vote in October's referendum.
Iraqi Defence Minister Saadun al-Dulaim said that the only people to die at the hands of insurgents were the seven killed in the mortar attack.
"What happened has nothing at all to do with any sectarian tension," he said live on Iraqi TV.
"People swarmed the bridge. There had to be a search operation at the end of the bridge, so crowds gathered and a certain scream caused chaos ... and this sorrowful incident took place."
Officials had earlier suggested that someone in the crowd deliberately triggered the stampede by saying they had seen a suicide bomber.
BBC Middle East analyst Roger Hardy says that, because of radical Sunni attacks on big Shia gatherings in the past, it was not unreasonable for the worshippers to be nervous.
Television pictures showed large crowds of Shia pilgrims heading towards the Kadhimiya mosque to mark the martyrdom of the 8th Century religious figure Imam Musa al-Kadhim.
During the crush, iron railings on the bridge leading to the shrine gave way and hundreds of people fell into the water.
The bridge links the staunchly Sunni area of Adhamiya on the east bank of the Tigris and the Shia area of Kadhimiya on the west bank.
The government has declared three days of mourning.