At least six people have been killed and up to 30 Sunni mosques targeted as a wave of retaliatory attacks swept across Iraq following the catastrophic bombing of one of the holiest shrines in Shia Islam.

One of the 17 mosques desecrated in Baghdad was set on fire, others were machine-gunned, according to reports from military leaders. Three of the dead are said to be spiritual prayer leaders, the rest were worshippers killed at prayer. One imam was also reported to have been kidnapped.

The targeting of sacred sites was condemned by Jalal Talabani, Iraq's Kurdish President, as a deliberate attempt to foment sectarian unrest as talks continue to form a functioning coalition government.

"We are facing a major conspiracy that is targeting Iraq’s unity," Mr Talabani said. "This new ugly crime comes as a warning that there is a conspiracy against the Iraqi people to spark a war among brothers. We should all stand hand-in-hand to prevent the danger of a civil war."

The renewed wave of targeted violence began at dawn this morning with the dual bombing of the Askariya shrine in the town of Samarra, 60 miles north of Baghdad, a bloodless but symbolic act of provocation which some experts said bares the hallmark of al-Qaeda.

Mouwafak al-Rubaiel, the Iraqi National Security Adviser, said that around ten militants wearing the uniforms of Special Forces overpowered guards before laying the explosives and escaping.

The 100-year-old gilded dome of the shrine, which holds the tombs of Imam Ali al-Hadi and his son Imam Hassan al-Askari - direct descendents of Muhammad - was blown apart. Mosaics covering the walls shattered, littering the complex with debris. Relics including ninth-century helmets and shields were buried and damaged, according to witnesses.

As thousands of angry Shia demonstrators gathered with flags and copies of the Koran outside the ruined shrine, Ibrahim al-Jaafari, the Shia Prime Minister, appealed for unity in a live television address and declared three days of mourning.

The Sunni Endowment, which oversees religious activity for Sunni Muslims in Iraq, and Moqtada al-Sadr, the prominent Shia cleric, were among the broad swath of religious and political leaders who joined the calls for calm.

Even the reclusive and ageing senior Shia cleric, Grand Ayatollah Ali al-Sistani made a rare, if silent, television appearance underlining the gravity of the crisis. He also appealed for restraint in a written statement as protesters outside his office in Najaf chanted: "Rise up! Take revenge!"

The near-universal calls for peaceful protests fell on deaf ears and the reprisals grew in ferocity as news of the blast spread.

Thousands of Shia Muslims to the streets of cities across Iraq baying for vengeance and denouncing the shrine bombers as "animals".

In Basra, which is under the control of British forces, rocket-propelled grenades were fired at a Sunni mosque and there were heavy exchanges of fire outside an Islamic Party.

In Diwaniya, a local government official said an al-Mahdi Army militiaman was killed in clashes with Sunni residents. Shops in many towns closed down for the official mourning period.

As the violence on the streets showed no signs of diminishing, political and religious leaders expressed fears that the resurgent violence was pushing Iraq nearer to the brink of civil war.

Hazim al-Naimi, a political scientist at Baghdad’s Mustansiriya University, said: "For the Shi’ites ... this is a major assault comparable to an attack on Mecca for all Muslims. It could push the country closer to civil war."

The United States was quick to recognise the significance of the attack and unequivocal in its condemnation. Zalmay Khalilzad, the US ambassador, and General George Casey, who commands the US forces in Iraq, said in a joint statement: "The Shrine should be rebuilt and the United States will contribute to its reconstruction."

"Given the historic, cultural, and religious importance of this shrine, this attack is a crime against humanity."

Ordinary Shias were dismayed. "Whoever did this are not human beings. They are less than animals," said Wuroud Kathim, 29, a computer specialist from Baghdad.

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