The crush caused a railing to collapse on a Baghdad bridge packed with Shia worshippers sending crowds tumbling into the Tigris River.

An Iraqi interior ministry source said most of the dead were women and children.

Survivor Fadhel Ali, 28, who jumped into the river to look for bodies, said: "We were on the bridge. It was so crowded. Thousands of people surrounded me.

"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: "There were huge crowds on the bridge and the disaster happened when someone shouted that there was 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."

At least six people also died after drinking poisoned juice and food they received around the mosque. Tensions had been running high after a mortar and rocket attack two hours earlier killed at least seven people and injured at least 40 nearby.

A police source said large crowds had been heading to the Kadhimiya mosque in the old district of north Baghdad for a religious ceremony when someone shouted that there was a suicide bomber among them.

"Hundreds of people started running and some threw themselves off the bridge into the river," he said. "Many elderly died immediately as a result of the stampede but dozens drowned, many bodies are still in the river."

The crowd was celebrating the martyrdom of Musa Al-Kadhim, a revered religious figure among Shias. After the disaster, thousands of people rushed to both banks of the river searching for survivors. Hundreds of men stripped down and waded into the muddy water downstream from the bridge trying to extract bodies floating in the water.

Television reports said about one million pilgrims from Baghdad and outlying provinces had gathered near the Imam Mousa al-Kadim shrine for the annual commemoration of the Shia saint's death.

Iraq's Prime Minister Ibrahim Jaafari immediately declared three days of mourning. Iraqis are currently preparing to vote on a proposed constitution for their country, with Shia and Sunnis sharply divided on its contents.

Today is the last day the majority of Iraqis can register to vote in October's referendum on a new constitution. Despite the draft constitution, there has been no sign of an easing in the insurgency waged by Sunni Muslims, dominant under Saddam, and international guerrillas inspired by Osama bin Laden.

The US-led coalition, which invaded Iraq in March 2003, has been battling insurgents while Iraqis have tried to form a new post-Saddam constitution and government.

Shia religious festivals have often been targeted for attack by Sunni extremists seeking to trigger civil war. In March last year, suicide attackers struck worshippers at the Imam Kadhim shrine and a holy site in Karbala, killing at least 181.

The head of the country's major Sunni clerical group, the Association of Muslim Sholars, said that the disaster was "another catastrophe and something else that could be added to the list of ongoing Iraqi tragedies."

"On this occasion we want to express our condolences to all the Iraqis and the parents of the martyrs, who fell today in Kazimiyah and all over Iraq," the cleric, Haith al-Dhari said.

Meanwhile, eyewitnesses said the town of Qaim, 200 miles north-west of Baghdad, was virtually deserted today after a day of heavy fighting between the pro-government Bumahl tribe and the pro-insurgent Karabilah tribe.

Iraqi officials said 45 people had died in the clashes, during which hundreds of residents fled their homes.

The US military said jets bombed the region around Qaim and destroyed houses used by "a known terrorist".

• The court martial of seven British Paras accused of killing an Iraqi civilian will begin on Monday, the MoD has said.

The group - serving and former soldiers from the 3rd Battalion the Parachute Regiment - are jointly charged with murder and a second joint charge of violent disorder. The charges relate to the death of Nadhem Abdullah following a roadside incident in May 2003 in Al U'Zayra in southern Iraq.