Hundreds of police and army troops fanned out in Baghdad on Thursday, setting up checkpoints and fortifying polling stations with barbed wire and blast barriers, two days ahead of a historic constitutional referendum.

    The large number of troops and policemen on the streets, combined with the scarcity of people and vehicles, gave the city a disquieting calm.

    Similar security precautions were in place across much of Iraq in anticipation of a spike in attacks by insurgents who want to derail Iraq's political process. Nearly 450 people have been killed in violence over the past 18 days.