At least five hundred people have been killed in a stampede in northern Baghdad, Iraqi officials say. The incident happened on a bridge over the Tigris River as hundreds of thousands of Shia pilgrims marched as part of an annual religious festival. It was sparked by rumours of a suicide bomber in a crowd. In the ensuing panic many pilgrims were crushed and some fell into the river. Mortar rounds had been fired into the crowd earlier, killing seven people. About 36 others were injured when four mortar round landed close to the Kadhimiya mosque. So far no group has said it carried out the attack.