Political and religious leaders in Iraq have appealed for calm after an attack on one of the holiest sites in Shia Islam, the al-Askari shrine in Samarra.
Blasts destroyed two minarets on the shrine, which houses one of the city's two tombs for revered Shia imams.
Iraq's most prominent Shia cleric, Grand Ayatollah Ali Sistani, condemned the attack but urged people not to respond with violence.
A 2006 bombing of the shrine's dome triggered a wave of sectarian violence.
Thousands of people have died in scores of attacks that have swept the country since then.
The BBC's Jim Muir in Baghdad says there are obvious fears this latest attack might give the sectarian violence further impetus.
Despite appeals for restraint, three Sunni mosques in the Baghdad area were reported to have been attacked shortly after the shrine bombing.
In the wake of the blasts in Samarra, a curfew was imposed on the city as Iraqi security forces and US troops rushed to the area.
In Baghdad, Prime Minister Nouri Maliki held emergency meetings with US and Iraqi security chiefs, as well as with the US ambassador.
Amid fears of an upsurge in violence, Mr Maliki's office announced an open-ended curfew in Baghdad, where extra troops flooded onto the streets.
In a speech on state TV, Mr Maliki called on Iraqis to "stand together against those who want to stir strife".
He blamed al-Qaeda in Iraq and supporters of former president Saddam Hussein for the blasts, and said he had ordered the arrest of all the security forces responsible for protecting the shrine.
A state of emergency has also been declared in Najaf, site of another important Shia shrine, where the radical Shia cleric, Moqtada Sadr, has declared a three-day period of mourning.
"What did the government do to protect the tombs?" he asked in a statement.
Mr Sadr also called for peaceful demonstrations to demand an end to the US-led occupation.
Later, the political bloc loyal Mr Sadr suspended participation in parliament, demanding that the government takes "realistic measures" to rebuild Shia and Sunni mosques.
The BBC's Andrew North in Baghdad says even if there is no renewed upsurge in sectarian bloodletting, the attack comes at a very bad time politically.
It is likely to make it even harder for the prime minister to implement reconciliation measures aimed at persuading Sunni groups to abandon the insurgency because of opposition from Shia political blocs on which he depends.
Samarra, a Sunni Muslim stronghold 100km (60 miles) north of Baghdad, has long been a centre of the armed insurgency against US troops and the Shia-dominated Iraqi administration.
Little remains of the minarets that stood either side of the shrine
The al-Askari shrine is of immense spiritual importance for Shia Muslims throughout the world and has attracted millions of pilgrims over the centuries.
Part of the Imam Ali al-Hadi mausoleum, the shrine contains the remains of the 10th and 11th imams, reputed to be direct descendants of the Prophet Muhammad.
Imam Ali al-Hadi died in 868 AD and his son, al-Hassan al-Askari, died in 874 AD.
The mosque's two minarets had escaped damage when its famous golden dome was destroyed by a huge explosion in February last year.
That attack was widely believed to be the work of Sunni militants from the al-Qaeda movement, some of whom were later arrested.
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