In Iraq's deadliest day since the US-led war of March 2003, hundreds of women, children and elderly people were trampled underfoot or jumped to their deaths from the bridge on Wednesday after a deadly mortar strike on a Shia shrine.
Iraq authorities said the tragedy - which risks inflaming sectarian tensions in the country - was a "terrorist" act by toppled ruler Saddam Hussein's loyalists and al-Qaida frontman in Iraq, Abu Musab al-Zarqawi.
A security official said 965 were killed and 465 injured in the crush of pilgrims who converged on the Kadhimiya mosque in northern Baghdad for a ceremony mourning the death of a revered Shia imam.
"We are expecting more drowned corpses to surface," he said.
Most were trampled to death or fell from Al-Aaimmah bridge into the Tigris river as panic gripped thousands of pilgrims among the several million attempting to make their way to the mosque.
The two-lane bridge was littered with hundreds of sandals lost in the pushing and panic, while children floundered in the muddy waters below, trying to reach dry land.
It was the single biggest confirmed loss of life in Iraq since the March 2003 invasion.
Prime Minister Ibrahim al-Jaafari, a Shia, declared a three-day mourning period.
Defence Minister Saadoun al-Dulaimi, a Sunni, said: "What happened has nothing at all to do with any sectarian tension.
Islamic historians say Imam Musa ibn Jaafar al-Kadhim was imprisoned and poisoned in Baghdad in 799 AD by Harun al-Rashid, the leader of the Muslim caliphate at the time, who feared Kadhim as a political rival for power.
On Wednesday a symbolic coffin for the imam was carried through the crowd, with groups of men banging drums in symphony. The crowds chanted and waved Iraqi and Shia flags.
Historians say Kadhim's body was dumped by the authorities on the Imamain bridge, the same bridge where hundreds of Iraqis died in the stampede.
Organisers said up to 250,000 pilgrims had made this year's annual trip to northern Baghdad's Kadhimiya shrine, a number that was lower than previous years' because of security fears.
"Only the seven that were killed this morning were killed by terrorists," he said, referring to seven killed by a mortar attack before the stampede.
He added that three bombers were stopped on Wednesday some distance from the shrine but "blew themselves up before reaching their destination".
"Pushing started when a rumour was spread by a terrorist who claimed that there was a person with an explosive belt, which caused panic," Interior Minister Bayan Jabor said.
The Baghdad marchers were commemorating the death in the year 799 of Imam Musa ibn Jaafar al-Kadhim, one of the 12 principle Shia saints who is buried in a mosque in the northern Baghdad neighbourhood of Kadhimiyah.
Television reports said about one million pilgrims from Baghdad and outlying provinces had gathered near the Kadhimiyah shrine.