Iraqi state television announced that the extraordinary curfew would order people off the streets of the capital until 4pm on Friday, preventing many residents from attending the week's most important Muslim prayer service.
The curfew will also apply to three other provinces including Samarra, where the bombing of a Shia shrine on Wednesday sparked two days of violence that has killed more than 130 people so far.
No one has claimed responsibility for the bombing of the 1000-year-old Imam Ali al-Hadi mausoleum, which provoked a wave of reprisal attacks on Sunni mosques nationwide and scores of killings.
Appeal for calm
In one of the worst single incidents, 47 civilians were shot dead at a makeshift checkpoint on Thursday and their bodies dumped in a ditch.
Tensions remain high. Both Shia and Sunni leaders appealed for calm, but blamed the violence on each other, each claiming they were the victim.
The Association of Muslim Scholars, a leading Sunni authority in Iraq, said at least 168 Sunni mosques had been attacked, 10 imams killed and 15 abducted since the Samarra attack.
The Interior Ministry said it could only confirm figures for Baghdad, where it had reports of 19 mosques attacked, one imam killed and one abducted.
Sectarian tensions run high even
after two days of violence
Spokesman Abd al-Salam al-Kubaisi blamed the violence on the country's leading Shia figure, Grand Ayat Allah Ali al-Sistani, and other religious leaders who called for demonstrations against the attack.
Muqtada al-Sadr, the Shia radical, told Aljazeera from Qom, Iran, that Sunnis should join Shia in pledging not to kill fellow Muslims to distance themselves from takfiris - Sunni extremists who target Shias.
'Voices of reason'
Ibrahim al-Jaafari, the Iraqi prime minister, had proclaimed three days of national mourning and decreed that Thursday would be a public holiday in a bid to get people off the streets.
Meanwhile, George Bush, the US president, denounced the bombing as a "political act" meant to sow "civil strife" and appealed for an end to the violence.
He said: "The voices of reason from all aspects of Iraqi life understand that this bombing is intended to create civil strife, that the act was an evil act."
Washington is seen as being keen to prevent an all-out sectarian war as it looks to extract about 130,000 US troops from Iraq.
Seven US soldiers died in a pair of roadside bombings north of Baghdad during the recent violence according to US military sources.