Iraq's top Shia religious leader Ali al-Sistani has called for restraint after the bombing of an important Shia mosque.
"He condemns the attack and urges calm and not to do acts of reprisal against Sunnis," Mr Sistani's spokesman, Hamed Khafaf, said.
Two minarets of a revered Golden Mosque in the Iraqi city of Samarra were blown up today, targeting a shrine bombed last year in an attack that sparked a wave of sectarian killing. Sunni or al-Qaeda militants are suspected of carrying out the attack.
Saleh al-Haidari, the head of the Shia endowment in Iraq
A senior Iraqi government official said the attack at Samarra's mosque was "very bad news for Iraq" while the US military expressed concern and said it was monitoring events. The minarets were destroyed, news photographs showed.
The bombing of the mosque last year, which wrecked the shrine's famous golden dome but did not damage the minarets, was a turning point for Iraq, unleashing violence that has killed tens of thousands of people and pushed the country to the brink of all-out civil war.
Fearing further bloodshed, the government said it would impose a total curfew on Baghdad from 3pm (Midday Irish time) until further notice.
"This is a criminal act which aims at creating sectarian strife," Saleh al-Haidari, the head of the Shia endowment in Iraq, a major religious body, said.
Shia officials blamed Sunni Islamist al Qaeda for the attack. It was unclear exactly how the minarets had been blown up, but residents said there had been clashes in the area between gunmen and police before the blast at around 9am.
The Shia group loyal to anti-American cleric Moqtada al-Sadr urged its supporters to remain calm and accused militants of planting explosives to bring down the minarets.
After the bombing in 2006 gunmen from the Mehdi Army militia, loyal to Sadr, targeted members of Iraq's Sunni Arab community in Baghdad in revenge attacks. The Golden Mosque is one of the four major Shi'ite shrines in Iraq.
Samarra, north of Baghdad, is a predominantly Sunni city. Other major sites are in the holy Shia cities of Najaf and Kerbala and the Baghdad district of Kadhimiya, also mainly home to Shias.
Two of the 12 revered Shia imams are buried in the Samarra shrine - Imam Ali al-Hadi, who died in 868 and his son, the 11th imam, Hasan al-Askari, who died in 874.