The two minarets of the al-Askari shrine in Iraq, one of the holiest sites in Shia Islam, have been destroyed by two explosions.
According to witnesses the minarets collapsed completely after being hit by bomb blasts at around 0900 (0500 GMT).
The shrine houses one of two tombs in Samarra for revered Shia imams.
The 2006 bombing of the shrine's dome is widely believed to have set off a continuing spiral of sectarian violence in which many thousands have died.
The BBC's Jim Muir in Baghdad says there are obvious fears this latest attack might give it yet further impetus.
The head of the Shia endowment foundation said the minarets had been blown up by "extremists".
"It is a terrorist attack aimed at sparking sectarian violence," Sheikh Saleh al-Haidari told the AFP news agency.
Iraq's most prominent Shia cleric, Grand Ayatollah Ali al-Sistani, condemned the attack as a "heinous crime" and urged people "not to follow the path of sectarianism", his office said.
Appeal for calm
Almost immediately after the explosions, a curfew was imposed on Samarra as Iraqi security forces and US troops rushed to the area.
Meanwhile, police in the shrine's compound reportedly fired into the air to keep away angry Shia demonstrating outside.
A team of explosives experts from the Iraqi police was also sent to the shrine to ascertain the cause of the blasts.
In the capital, Baghdad, Prime Minister Nouri Maliki held emergency meetings with US and Iraqi security chiefs, as well as with the US ambassador.
His office then announced an open-ended curfew in Baghdad too, fearing a possible upsurge in sectarian violence.
Extra troops flooded into the streets, but angry Shia militiamen were ahead of them.
Iraqi police said a Sunni mosque in the east of the city was demolished by explosives.
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, but added that 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 to Mr Sadr suspended participation in parliament, demanding that the government takes "realistic measures" to rebuild Shia and Sunni mosques.
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 which 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.
Our correspondent says there are bound to be questions about how such an obvious and significant target could have been attacked again.