An Interior Ministry source said most of the casualties on Wednesday were women and children and most died by drowning or being trampled on.

He put the toll at 647, with 301 injured, as of 2.15 pm (1015 GMT).

A railing on the bridge collapsed sending crowds tumbling into the Tigris River.

Television pictures and reports said about one million pilgrims were heading to the Imam Musa al-Kadhim shrine in the city's heavily Shia al-Kadhimiya district on Wednesday to commemorate the death of imam Musa al-Kadhim, a revered religious figure among the Shia. 

There was confusion over what caused the railing to collapse, but police said there were unconfirmed rumours that the stampede may have been caused by someone in the crowd shouting that there was a bomber in the crowd.

Rumours of bombers caused a
panic and stampede

"Dozens of pilgrims fell in the river Tigris as they panicked following rumours of the presence of two suicide bombers in the crowd, while they were crossing al-Aaimmah bridge near the mosque," a source said. 

Iraq's Deputy Health Minister Jalil al-Shammari confirmed that hundreds had been killed.

One hospital said it had received at least 100 bodies by 12.30pm. The hospital source said bodies were being sent to two other nearby hospitals as well. 

"We have lost count, we have hundreds and hundreds of dead
and injured," a Health Ministry official said.

"We can't tell how many are dead. Many bodies are still in
the river," the official added.

Mortar attack

"We have lost count, we have hundreds and hundreds of dead
and injured"

Health Ministry official

Tensions were running high after a mortar and rocket attack more than two hours earlier killed at least seven people and injured at least 40 near the mosque.

A medical officer at Baghdad's al-Kadhimiya hospital said half of the wounded in the mortar attack were women.

US Apache helicopters fired on the attackers who launched the rockets, a US statement said.

Six other people were wounded in a separate attack when attackers opened fire on Shia pilgrims in Baghdad's al-Adhamiya neighbourhood, an Interior Ministry source said.

The Iraqi government declared a three-day mourning for the stampede and bridge collapse victims.