BAGHDAD, Iraq - At least 648 people were killed in a stampede on a bridge today when panic engulfed a Shiite religious procession amid rumors that a suicide bomber was about to attack, officials said. It was the single biggest confirmed loss of life in Iraq since the March 2003 invasion.

Scores jumped or were pushed to their deaths into the Tigris River, while others were crushed in the crowd. Most of the dead were women and children, Interior Ministry spokesman Lt. Col. Adnan Abdul-Rahman said.

Tensions already had been running high in the procession in Baghdad's heavily Shiite Kazimiyah district because of a mortar attack two hours earlier against the shrine where the marchers were heading. The shrine was about a mile from the bridge.

Abdul-Rahman said 648 were killed and 322 injured, with survivors rushed in ambulances and private cars to several hospitals, where officials scrambled to compile accurate casualty figures.

Prime Minister Ibrahim al-Jaafari, a Shiite, declared a three-day mourning period.

Thousands of people rushed to both banks of the river to search for survivors, and bare-chested men jumped in to try to recover bodies.

Scores of bodies covered with white sheets lay on the sidewalk outside one hospital because the morgue was jammed. Many of them were women in black gowns, as well as children and old men.

Sobbing relatives wandered amid the bodies, lifting the sheets to try to identify their kin. When they found them, they would shriek in grief, pound their chests or collapse to the ground, sobbing.

Hundreds of thousands of Shiites had been marching across the bridge, which links a Sunni and Shiite neighborhood, heading for the tomb of Imam Mousa al-Kadhim, a 9th century Shiite saint.

Television reports said about 1 million pilgrims from Baghdad and outlying provinces had gathered near the shrine in the capital's Kazimiyah district for the annual commemoration of the saint's death. The shrine is about a mile from the bridge.

"We were on the bridge. It was so crowded. Thousands of people were surrounding me," said survivor Fadhel Ali, 28, barefoot and soaking wet. "We heard that a suicide attacker was among the crowd. Everybody was yelling, so I jumped from the bridge into the river, swam and reached the bank. I saw women, children and old men falling after me into the water."

Health Minister Abdul-Mutalib Mohammed told state-run Iraqiya television that there were "huge crowds on the bridge and the disaster happened when someone shouted that there is a suicide bomber on the bridge."

"This led to a state of panic among the pilgrims and they started to push each other and there was many cases of suffocation," he said.

Shiite processions, which can draw huge crowds, are often targeted by Sunni extremists seeking to trigger sectarian war, so worshippers are on guard for trouble.

First reports suggested that the bridge's railing collapsed, but TV video showed the green, waist-high railing undamaged.

Mortar shells had exploded in the shrine compound about two hours earlier, killing at least seven people. U.S. Apache helicopters fired at the attackers.

Shiite religious festivals have often been targeted for attack by Sunni extremists seeking to trigger civil war among the rival communities.