A day of Shia religious ceremonies marking the death of a revered imam turned into Iraq's worst nightmare when throngs of pilgrims took fright after a deadly mortar attack on a shrine and rumours of suicide bombers in their midst.

A total of 647 people were drowned, trampled or crushed to death as they surged across Al-Aaimmah bridge near the Kadhimiyah mosque in northwest Baghdad, many of them falling into the Tigris river below.

"The crowd started to panic and women and children were being trampled underfoot," said Abdul Walid, 54, lying dazed on the floor of a nearby hospital.

"My son was on my shoulders, I don't know where he is now -- everybody was suffocating to death so I eventually had to jump."

In the middle of the bridge, the waist-high concrete barriers designed to foil car bombers were stained with the blood of victims, mostly women and children, crushed to death in the panic.

Covered in bandages and nursing a broken leg, Walid tearfully recounted the tragic events which led to Iraq's deadliest single incident since the 2003 US-led invasion.

"Even before we heard the rumours, the crowds had come to a standstill -- there was no more room to move and people couldn't breathe," said Walid.

With the atmosphere already tense after four mortars had slammed into the crowds at the Kadhimiyah mosque, killing seven, the worshippers became terror-stricken by reports of suicide bombers and a much larger tragedy unfolded.

Ahmed Jasim, 28, said he was in the middle of the bridge as huge crowds converged from both ends, some leaving the mosque and others making their way there.

"I was trying to pick up the children but was swept off my feet," said Jasim, whose shoulder and arm were fractured.

"I'm young and strong so I managed to force my way to the side and jumped into the Tigris -- another five minutes and I would have been crushed to death."

Several cases of criminal poisoning were also reported and witnesses said Shia militia members used loudspeakers to urge people not to drink water they had purchased near the scene.

Baghdad's hospitals were overwhelmed with the sheer numbers of casualties, with doctors losing count of the corpses passing through their wards.

"Most people are suffering from blunt trauma and the majority are women and children," said Dr Mazen Abdulilla, adding that medical services had been anticipating trouble at the event but were still struggling to cope.

"We have a shortage of painkillers and we still can't identify most of the bodies," Abdulilla said.