Most of the dead were crushed or suffocated, witnesses said, but many also drowned after falling or jumping into the Tigris River after the panicking crowd broke through the bridge's railings. The disaster was by far the greatest one-day loss of life since the U.S.-led invasion in March 2003.

Fear had begun spreading in the crowd an hour earlier, after a group of insurgents fired rockets and mortars near the gold-domed Shiite shrine where the pilgrims were headed, killing at least seven people and wounding two dozen.

Insurgents have often struck at Shiite religious processions in the past. But the stampede appears to have started with unfounded rumors of a man wearing a suicide belt on the bridge.

The pilgrims were among a throng of hundreds of thousands of mostly poor Shiites from northern Baghdad and the surrounding area who had converged on the shrine bearing colored banners and symbolic coffins to mark the anniversary of the death of Imam Musa al-Kazim, one of Shiite Islam's holiest figures.

''We were all chanting slogans about Imam Musa, and then people started shouting about a suicide bomber,'' Waleed Hameed Andul al-Radha said as he lay on a cot at Kindi Hospital with a chest injury, after removing an oxygen mask to speak. ''They started crashing into each other; no one would look back or give a hand to help the ones who had fallen. People started running on top of each other, and everyone was trying to save himself.''

In the aftermath of the stampede, with some pilgrims continuing their procession, black-clad women keened over dead bodies in the streets of Kazimiyah, the Shiite neighborhood where Imam al-Kazim's shrine is situated. On the bridge itself, hundreds of the victims' sandals and shoes had been swept into piles.

Local hospitals were overwhelmed, their floors lined with dead bodies, including many women and children, some drenched in river water. Relatives of the victims streamed in and out, some of them pulling up the sheets on dozens of bodies until they recognized one, and then bursting into wails of grief.

There were reports in Baghdad's hospitals that some pilgrims had died in a mass poisoning. But Health Ministry officials said they could not confirm any poisonings. Shiite Muslims believe that Imam al-Kazim was poisoned by agents of Harun al-Rashid, the Sunni caliph, in the late eighth century, and history often merges with the present among religious pilgrims here.

The Iraqi authorities had blocked off roads to car traffic throughout northern Baghdad starting Tuesday evening, anticipating attacks on the hundreds of thousands of Shiites who were converging on the capital. The bridge where the stampede took place marks an especially fragile fault line, linking Kazimiyah with Azamiyah, a Sunni area that has long been a stronghold of support for Saddam Hussein and the insurgency.

The disaster came at a time of high sectarian tensions, four days after the new draft constitution was presented to Iraq's Parliament over the angry objections of Sunni representatives. Many leading Sunnis have called for voters to reject the document when it goes before a nationwide referendum in October, and there have been demonstrations against the charter by Sunnis in central and northern Iraq.

Also on Wednesday, an official with the Iraqi Special Tribunal announced that Oct. 19 will be the start date for the trial of Saddam and three of his top officials. That trial, to be presided over by five judges, could also prove as divisive as the constitutional referendum, especially after several recent demonstrations by Sunni Arabs chanting their loyalty to the former president.

Prime Minister Ibrahim al-Jaafari, a Shiite, appeared on television to declare a three-day national mourning period for the victims of the stampede.

Iraq's most revered Shiite religious figure, Grand Ayatollah Ali al-Sistani, issued a statement calling for an investigation and putting the blame for the stampede on ''terrorists.'' Other Shiite figures placed the blame more squarely on insurgents, as did Iraq's Shiite interior minister, Bayan Jabr. But it was too early to say how the Shiites, who have shown remarkable restraint in the face of attacks by Sunni insurgents in the past, would respond to the stampede.

For their part, leaders of Iraq's Sunnis and Kurds released statements on Wednesday calling for calm and sending condolences to the victims and their families. The leader of the Iraq Islamic Party, the country's best-known Sunni political group, singled out the Sunni residents of Azamiyah, calling on them to help out in the disaster.

For some Iraqi leaders, the disaster said more about bad planning than sectarian agendas. Iraq's Shiite health minister, Abdul Muqtalib Ari Muhammad, issued an angry call for the ministers of interior and defense - a Shiite and a Sunni, respectively - to take full responsibility for the disaster or resign. Speaking at a news conference, he said the ministers should have better secured the roads leading to the shrine.

Ceremony disasters

Deadly incidents connected to Islamic religious ceremonies:

* Shiite worshippers stampede on a Baghdad bridge during commemoration of a Shiite saint's death Wednesday.

* A suicide bomber blows himself up at a Shiite mosque during a funeral in the northern city of Mosul on March 10, 2005.

* Car bombs tear through a Najaf funeral procession and Karbala's main bus station on Dec. 19, 2004.

* Coordinated blasts from suicide bombers, mortars and planted explosives strike Shiite Muslim shrines in Karbala and Baghdad on March 2, 2004.

* Pilgrims stampede in the Saudi Arabian city of Mina during ''stoning of the devil'' ritual at the annual pilgrimage on Feb. 1, 2004.

* A car bomb explodes outside a mosque in Najaf on Aug. 29, 2003.

* Gunmen in the Pakistani city of Quetta storm a Shiite mosque packed with prayer worshippers on July 4, 2003.

* Hajj pilgrims stampede during ''stoning of the devil'' ritual in Mina on March 5, 2001.

* Several pilgrims fall off an overpass during ''stoning of the devil'' ritual in Mina, causing widespread panic on April 9, 1998.

* Fires driven by high winds tear through a sprawling, overcrowded tent city at Mina on April 15, 1997.

* Pilgrims stampede in Saudi Arabia's holiest city, Mecca, as worshippers surge toward a cavern for the symbolic ritual of ''stoning the devil'' on May 23, 1994.

* Pilgrims stampede in an overcrowded pedestrian tunnel leading to holy sites in Mecca on July 9, 1990, in the worst hajj-related tragedy.

* Security forces clash with Iranian pilgrims staging an anti-U.S. demonstration in the Saudi city of Mecca on July 31, 1987.

* A Pakistani jetliner carrying hajj pilgrims catches fire soon after takeoff from Jiddah to Riyadh, the Saudi capital, and breaks apart after an emergency landing on Aug. 3, 1980.