Nearly 650 Iraqi Shiites died in a stampede over a Tigris River bridge in Baghdad on Wednesday, panicked by rumors a suicide bomber was about to blow himself up, an Interior Ministry source told Reuters.
The source said most of the dead were women and children and "died by drowning or being trampled on".
By 2:15 pm the death toll had risen to 647, with 301 injured, the source said.
Television images showed people clambering down from the bridge to escape the surging crowd and piles of slippers left behind after panic swept the throng.
One hospital said it had received at least 100 bodies by 12:30. A hospital source said bodies were also being sent to two nearby hospitals.
A police source said swarming crowds had been heading to the Kadhimiya mosque in the old district of north Baghdad for a religious ceremony when someone yelled there was a suicide bomber among them.
"Hundreds of people started running and some threw themselves off the bridge into the river," the source said.
"Many elderly died immediately as a result of the stampede but dozens drowned, many bodies are still in the river and boats are working on picking them up."
Earlier at least seven people were killed in three separate mortar attacks on the crowd as thousands of people marched to the mosque, reports Reuters.
According to CBC, Police say the stampede may have been caused when rumors spread that a suicide bomber was prepared to blow himself up.
Television reports said about one million pilgrims from Baghdad and region had gathered near the Imam Mousa al-Kadim shrine in the capital's Kadhimiya district. They were there for the annual commemoration of the death of the Shia saint.
Tensions have been high between religious and ethnic communities leading up to a referendum on a new constitution for Iraq.
Parliament completed work on a draft constitution on Sunday. It must be approved by a popular mandate before Oct. 15 to come into force.