BAGHDAD, Iraq — Rumors of a suicide bomber sowed panic among thousands of Shiite Muslim pilgrims on a bridge yesterday, triggering a stampede that killed about 800 people, many of whom plunged to their deaths in the Tigris River.

It was Iraq's single largest loss of life since the U.S. invasion in March 2003.

Most of the dead were women, children and the elderly, Iraqi officials said. More than 323 people were injured.

In a twist for a nation increasingly ravaged by sectarian rivalries, survivors and rescuers credited residents of a hard-line Sunni Arab neighborhood with helping to save the lives of countless Shiites.

Witnesses said Sunnis from the Adhamiyah district waded into the Tigris to pull Shiites from the water and helped haul them to safety.

Adhamiyah, where Saddam Hussein made his last public appearance after the invasion, was one of the last neighborhoods in Baghdad to stop fighting occupation forces in 2003 and remains a stronghold of Baath party loyalists. "Adhamiyah saved them," said Ahmed Abdullah Hussein, 62, an auto mechanic.

Some rescuers said most victims died on the bridge from suffocation. "Their faces were black, black like this tire," said Hussein, a Sunni, at his repair shop in the shadow of the bridge.

Reflecting the confusion of the day, casualty figures from various government agencies varied widely. The Health Ministry said 769 people were killed and 307 wounded, while the Interior Ministry put the figure at 844 dead and 458 injured.

The country's biggest Shiite party gave figures of 759 dead and 300 wounded. Other reports estimated the death toll would climb to more than 1,000.

Tensions had been high among the pilgrims, who were marching to Baghdad's Kadhimiya shrine, because of an insurgent rocket and mortar barrage earlier in the day that killed seven worshippers.

Survivors and security officials placed much of the blame for the stampede on security checkpoints set up at the entry to the bridge, narrowing foot traffic to one or two people at a time so that guards could search male pilgrims for explosives.

Police later said they found no explosives at the bridge, either on any individual or in any cars parked nearby. Instead, poor crowd control and the climate of fear in Iraq after years of bullets, bombings and bloodshed appeared largely to have caused the carnage.

Marchers jammed up at a checkpoint at the western edge of the Two Imams bridge, which has been closed to civilians for months.

"This tragedy was the direct result of terrorism; hundreds of innocent people, mostly women and children, have died because of the fear and panic that terrorists are sowing in Iraq," NATO Secretary-General Jaap de Hoop Scheffer said in a statement.

Defense Minister Saadoun al-Dulaimi, a Sunni, said three suicide bombers were stopped yesterday some distance from the shrine but "blew themselves up before reaching their destination."

State Department spokesman Sean McCormack said in Washington that he was not aware of any evidence that the stampede on the bridge was caused by a suicide bombing.

The marchers were commemorating the death in the year 799 of Imam Moussa ibn Jaafar al-Kadhim, one of the 12 principal Shiite saints who is buried in a mosque in the northern Baghdad neighborhood of Kazimiyah.

Since the 2003 ouster of Saddam, a Sunni, the Shiite political parties have encouraged huge turnouts at religious festivals to display the majority sect's power in the new Iraq. Sunni religious extremists have often targeted the gatherings to foment sectarian war, but that has not stopped the Shiites.

Mariam Abbas, 22, survived the crush on the bridge. "You felt your body would collapse, you could not breathe," she said. Abbas said she clasped the shoulders of a young man beside her in a desperate attempt to lift herself high enough out of the throng to gasp air.

"Some people jumped from the bridge: old men, young people, kids," she said.

As Abbas began losing consciousness, she said she felt the grasp of a stranger.

"He grabbed me by my arms and held me up, and threw me down to his friends below the bridge," Abbas said. Her Sunni rescuers gently splashed water on her face to revive her. "They said, 'Are you OK?' And then they ran to help others."

Adhamiyah residents took hundreds of others, most of whom were mortally injured, to a Sunni mosque, a Sunni hospital and to Sunni homes. Bystanders brought corpses to hospitals overflowing with the dead; many bodies were placed outside on sidewalks.

Prime Minister Ibrahim al-Jaafari, a Shiite, declared three days of national mourning for the victims.

Government ministers traded blame for the disaster, with some suggesting the tragedy was caused by insurgent sabotage. Al-Jaafari cited rumors that pilgrims had been poisoned by juice given to them as they walked through the Sunni district.

Throughout the day, crowds estimated at more than 1 million streamed toward the shrine.

The stampede began near the east end of the bridge, which links Adhamiyah to a Shiite neighborhood on the other side of the river. Crowds going to the shrine were backed up at the concrete checkpoints, while throngs of those returning from the shrine tried to push past in the other direction.

"More than one person started yelling and saying, 'The bridge will fall down, the bridge will explode,' " said Khalid Fadhil, a goldsmith who witnessed the stampede. "So the people started running in panic, pushing each other, trying to run away.

"Some of the people fell down, and the people stepped on them. The others threw themselves off the bridge, into the river," Fadhil said.

Some of the pilgrims at midspan plunged more than 90 feet into the Tigris.

"Whoever was able to swim and knew how to swim survived. The people who didn't know how died," said Sattar Jabbar, 22, a member of the Mahdi Army, the militia loyal to Shiite cleric Muqtada al-Sadr, which helped provide security for the pilgrimage.

Others closer to the end of the bridge who jumped slammed to their deaths on the ground below.

"I saw an old woman who was completely panicked and crying throw herself from the bridge," Fadhil said. "I saw another man fall on the bricks of the shore and die immediately. I saw seven people were brought dead near the end of the bridge, smothered."

Rubber sandals, bundles of food and knots of twisted clothing collected, along with bodies, on the playground and the concrete beneath the bridge. Shoes piled up in layers on the bridge above, their owners injured or dead.

The swirl of pilgrims being crushed underfoot and bodies slipping from the bridge stretched on for several minutes, witnesses said.

Two hours earlier, several mortar and rocket rounds had hit the area around the shrine, killing seven people and injuring at least 40.

U.S. Apache helicopters fired on the attackers, the military said in a statement.

By late afternoon, security workers on the bridge restored an orderly flow of pedestrians coming and going from the shrine.

"Too late," said one pilgrim, who identified himself only as Hussein. "After the disaster they did this."

Material from The Washington Post, Reuters, the Chicago Tribune and The Associated Press was included in this report.