This combination of 3 images shows the stages of destruction of the Askariya Shrine in Samarra, Iraq. From top to bottom: a Feb. 2004 photo of the shrine, the shrine in Feb. 2006 following an explosion which destroyed its dome, and a Wednesday, June 13, 2007 view after insurgents blew up its two minarets. Saboteurs blew up the two minarets of a revered Shiite shrine in Samarra early Wednesday, in a repeat of the 2006 attack that shattered its famous golden dome and unleashed a wave of retaliatory sectarian violence that still bloodies Iraq. [AP]

The assault on the Askariya Shrine, one of the holiest in Shiite Islam, immediately stirred fears of a new round of intra-Muslim bloodshed, and prompted the 30-member bloc of radical Shiite cleric Muqtada al-Sadr to suspend its membership in Iraq's parliament, threatening a deeper political crisis.

To ward off a surge of violence, Prime Minister Nouri al-Maliki quickly imposed an indefinite curfew on vehicle traffic and large gatherings in Baghdad. Before the curfew took hold, arsonists set fire to a Sunni mosque in western Baghdad, police said.

A Shiite shrine was also blown up north of Baghdad, while two Sunni mosques were bombed south of the capital, police said. One was destroyed and the other lost its minaret.

Iraq's top Shiite cleric, Grand Ayatollah Ali al-Sistani, called on "believers to exercise self-restraint and avoid any vengeful act that would target innocent people or the holy places of others."

It wasn't clear how the attackers evaded the shrine's guards to mount the stunning operation, detonating the blasts around 9 a.m., and bringing down the two slender golden minarets that flanked the dome's ruins at the century-old mosque. No casualties were reported.

Policemen at the shrine were subsequently detained and will be questioned as part of the investigation, al-Maliki said. Later, the Interior Ministry said members of "a terrorist group" had been arrested and were being interrogated. The statement did not elaborate.

An official close to the prime minister, citing intelligence reports and speaking on condition of anonymity because of the sensitivity of the information, said the bombing was likely the work of al-Qaida, whose militants have recently moved into Samarra from surrounding areas.

In a conference call with reporters, Brig. Gen. Kevin J. Bergner was asked about unconfirmed reports of a skirmish among Iraqi security forces before the attack, which may have been somehow related to the bombing.

"It's unclear at what point relative to the explosions that happened, but that's exactly what the (Iraqi) investigation will build the best possible summary of," the U.S. military spokesman said.

In a nationally televised address, al-Maliki said he had ordered security forces to bolster protection of Iraq's other religious shrines and mosques.

His office also said he met with the U.S. commander, Gen. David Petraeus, and U.S. Ambassador Ryan Crocker to ask that U.S. reinforcements be sent to Samarra, 60 miles north of Baghdad, and that U.S. troops in the capital go on heightened alert.

A few hundred U.S. soldiers are stationed around Samarra to provide security, although they rarely enter the shrine's perimeter and leave protection of the mosque to Iraqi forces.

The U.S. command had no immediate comment on military moves. Crocker and Petraeus later released a statement calling the attack and "act of desperation" and "a deliberate attempt by al-Qaida to sow dissent and inflame sectarian strife among the people of Iraq."

Lt. Col. Christopher Garver, another U.S. military spokesman, said the command was "obviously very concerned about this and our primary goal is to prevent any violence of the kind that broke out after the last bombing."

The carefully orchestrated 2006 explosion enraged Shiites, who ignored appeals for calm and attacked Sunni clerics and mosques. Nearly 140 people were killed the next day.

In neighboring Shiite Iran, which has been accused of funding and arming Shiite militias in Iraq, President Mahmoud Ahmadinejad blamed U.S. forces for failing to prevent the mosque attack, and threatened to halt regional cooperation to stop Iraq's spiraling violence.

The powerful blasts shook Samarra, sending a cloud of dust into the air, said Imad Nagi, a storeowner 100 yards from the shrine. "After the dust settled, I couldn't see the minarets anymore. So, I closed the shop quickly and went home," he said.

Police in the area around the shrine began firing into the air to keep people away, witnesses said, and Iraqi army and police reinforcements poured in. The Interior Ministry said a national police force was ordered to move immediately to Samarra.

A U.S. military official in the area, speaking on condition of anonymity because he wasn't authorized to release the information, said Samarra remained calm.

But a Samarra resident, Abdul-Khali Mohammed, predicted violence in the capital: "The Shiite militias now will seize this opportunity to kill Sunni families in Baghdad."

In the Baiyaa area of the capital, insurgents set fire to the Sunni Khudair al-Janabi mosque, police and witnesses said. A sole guard escaped, and the mosque was empty at the time.

In Khalis, 50 miles north of the capital, police said insurgents planted explosives inside the Shiite shrine of Imam Ali Kamal, destroying the building completely.

The reaction was swift in Shiite-dominated southern Iraq. Black banners were hoisted outside the Najaf residence of radical cleric al-Sadr, who called for three days mourning and peaceful demonstrations to mark the minarets' destruction and criticized the government for not doing enough to protect the site.

He also said the U.S. occupation is "the only enemy of Iraq" and "that's why everyone must demand its departure," or a timetable for its departure.

Later, in Baghdad, the 30 members of the Sadrist bloc in parliament issued a statement saying they were boycotting parliament until the government takes "realistic" steps to rebuild the Askariya shrine. The move by the Sadrists, whose support for al-Maliki has recently waned, is likely to weaken the Shiite-dominated government and delay adoption of a series of laws needed to build national reconciliation in Iraq.

The Askariya shrine's dome was destroyed on Feb. 22, 2006, in a bombing blamed on Sunni Muslim militants believed linked to al-Qaida. The mosque compound and minarets had remained intact but closed afterward.

The mosque contains the tombs of the 10th and 11th imams ¡ª Ali al-Hadi, who died in 868, and his son, Hassan al-Askari, who died in 874. Both are descendants of the Prophet Muhammad, and Shiites consider them to be among his successors.

The shrine also is near the place where the 12th imam, Mohammed al-Mahdi, disappeared. Al-Mahdi, known as the "hidden imam," was the son and grandson of the two imams buried in the Askariya shrine. Shiites believe he will return to Earth restore justice to humanity.

After last year's bombing, the mosque was guarded by about 60 Federal Protection Service forces and 25 local Iraqi police who kept watch on the perimeter, according to Samarra city officials. U.S. officials and others had promised to help rebuild the landmark dome, completed in 1905, but no rebuilding has begun.

In northern Iraq, meanwhile, militants blew up part of a bridge in the country's fourth attack on a span in as many days, police said. The attackers planted explosives under the Zikaytoon overpass near Kirkuk, about 180 miles north of Baghdad, said police Brig. Sarhat Qader. No one was wounded, he said.