By Jim Muir
BBC News, Baghdad
The latest attack on the al-Askari shrine in Samarra, one of the most revered sites in Shia Islam, has sent Iraq into an instant state of national alert.
A curfew was imposed soon after the attacks
Iraqi police reinforcements and US troops were rushed to Samarra itself, where a curfew was imposed almost as soon as the dust had settled over the battered shrine.
In Baghdad, Prime Minister Nouri Maliki held emergency meetings with his top security chiefs and conferred with the US ambassador and multinational forces commander.
The prime minister's office then announced that an open-ended curfew was also being ordered in the capital from 1500 local time.
A state of emergency was also reported at another major Shia centre, Najaf, to the south of Baghdad.
Calls for restraint
Shia militiamen, blamed for a wave of sectarian reprisals after the 22 February attack at Samarra in 2006, were reported to be out on the streets in force in many parts of Baghdad.
Loudspeakers at mosques in Sadr City, the teeming east Baghdad suburb where the Mehdi Army militia is strong, began broadcasting chants of "Allahu Akbar!" - "God is Great".
The movement's leader, the maverick young cleric Moqtada Sadr, called for three days of mourning and issued an appeal for calm and restraint.
So too did Ayatollah Ali Sistani, the religious eminence who is usually regarded as the most senior clerical figure among Iraq's Shia Muslims.
But similar appeals in the wake of the Samarra attack last year did not stop the wave of sectarian revenge killings against the Sunni community, for which the Mehdi Army has largely been blamed.
The first Samarra bombing was a watershed moment in the Iraqi crisis, triggering a spiral of violence that has taken thousands of lives among both Sunnis and Shia, and has proven almost impossible to stifle.
Despite all the precautions and the calls for restraint, there were predictions that the latest attack might add further fuel to the flames.
There are many unresolved flashpoints between the Sunni and Shia communities in Baghdad and elsewhere
"Even if Moqtada Sadr appears waving a copy of the Koran, it is 90% cent sure there will be violence," said one Baghdad Shia gloomily.
The fallout will clearly be a major challenge to the current security "surge" by thousands of US and Iraqi troops.
The level of US forces in Iraq is expected to reach its peak in the coming days, with the aim of stabilising the capital and other troubled areas in advance of an eventual coalition withdrawal.
In the hours after Wednesday's Samarra explosions there were unconfirmed reports that a Sunni mosque in east Baghdad had been burned, and a Sunni neighbourhood in the western part of the capital attacked by Shia militiamen.
There are many unresolved flashpoints between the Sunni and Shia communities in Baghdad and elsewhere, despite a process of sectarian separation that has seen hundreds of families from both sides displaced by campaigns of threats and violence.
Even if Moqtada Sadr is sincere in his calls for his Shia followers not to fall into the trap of launching sectarian reprisals, there has been a growing question over the extent to which he is really in control of the Mehdi Army.
It is widely reported to have split into several factions and fragments.
Whoever was responsible for the latest attack at Samarra - already blamed on radical Sunni insurgents - clearly knew exactly what they were doing and what the likely response would be.