BAGHDAD, Iraq – One of the most revered shrines in Shiite Islam was bombed early yesterday morning north of Baghdad, causing the collapse of its dome, police and eyewitnesses said. There was no immediate estimate of casualties in the latest in a series of sectarian attacks in the country.

The attack on the Al-Askari shrine, also known as the Golden Mosque, in the city of Samarra sparked immediate and widespread protests among Shiites across Iraq and reports of reprisal attacks on Sunni mosques. The blast appeared designed to further inflame sectarian tension between Iraq’s Shiite majority and the Sunni Arab population from whose ranks the bulk of the country’s insurgency is drawn.

“The main aim of these terrorist groups is to drag Iraq into a civil war,” Iraq’s national security adviser, Mowaffak al-Rubaie, said in an interview on a Dubai-based Arabic news channel, al-Arabiya.

Thousands of Shiite militia fighters – many armed with pistols, automatic rifles, and grenade launchers – took to the streets of cities across Iraq after the bombing, even as Shiite political and religious leaders called for peaceful demonstrations and restraint.

In Baghdad, Shiite militia fighters converged upon at least one Sunni Arab mosque and the headquarters of the Sunni-led Iraqi Islamic Party, witnesses said. Gunfire broke out at both sites, sending families in the neighborhood diving to the floors of their homes to escape bullets. American military helicopters backed Iraqi security forces as they tried to get the Shiite militia fighters there to withdraw.

Gunfire and bombings were reported at other Sunni mosques and political headquarters in Samarra, Basrah, and elsewhere.

The shrine in Samarra, a predominantly Sunni Arab city 65 miles north of Baghdad, contains the remains of two of Shiite Islam’s most prominent imams. The bomb is believed to have been planted a day earlier, Captain Basheer Qadoori of the city’s police force said.

“Last night, five armed men wearing ski masks broke into the shrine, kidnapped five guards of the shrine and planted two bombs inside,” Mr. Qadoori said.

An aerial photograph released by the American military showed the 33-foot-wide dome reduced to a shell of brown masonry and twisted iron, with nearby buildings also wrecked, the Reuters news agency reported.

Iraq’s most notable Shiite religious authority, Grand Ayatollah Ali al-Sistani, announced a week-long mourning period and urged people to go to the streets in “peaceful demonstrations to denounce this criminal act.” Ayatollah Sistani’s office said a detailed statement would be released later yesterday.

Prime Minister al-Jaafari, a Shiite, announced a three-day mourning in a televised appearance. “I call on my people to express their condemnation,” Mr. Jaafari said, asking Iraqis to “close the door to all those who are fishing in the troubled water.”

In Samarra, witnesses said that Interior Ministry commandos and Iraqi police were cordoning the shrine before the explosions took place. A resident who lives near the shrine, Ahmed Abdul Ghafour, 30, said: “I was leaving my house to go to work at 6 a.m., but the commandos did not allow me and said curfew is imposed. About an hour later, we heard the explosions.”

Residents and clerics in Samarra expressed their anger and took to the streets in protest. Sunni imams – religious leaders – used loudspeakers to urge people to demonstrate in front of the shrine peacefully. But Sunni demonstrators were dispersed by Iraqi police who shot in the air. Two people were wounded during the protests.

A Sunni cleric and a member of the Muslim Scholars Association in Samarra, Ahmed al-Dhaye, denounced the act and said: “We blame the Interior Ministry and the U.S. forces for failing to protect this holy shrine. We and all the people in Samarra are very enraged against this crime, as we never expected this to happen.”

Protests were also reported in the southern cities of Basra, Najaf, Hilla, and Karbala, where Shiites predominate. A prominent Sunni politician, Tariq al-Hashimi, said 29 mosques had been attacked across the country.

In Baghdad, news of the mosque bombing prompted thousands of Shiite men to abruptly leave their homes and jobs to report to offices of Shiite cleric Muqtada al-Sadr. School boys, fighting-age men, and graying elders marched on foot or piled into buses and the back of pickup trucks, to report for action.

“We are waiting for orders from our clerics,” men shouted outside the headquarters of Mr. Sadr, in Baghdad’s Sadr City.

Over loudspeakers late in the day, a preacher at the Sadr headquarters blamed the bombing on Iraq’s “occupiers,” meaning Americans, and ordered that there be no retaliation against Sunnis.

“There must be no attack on any mosque or church,” cleric Abdul Zara Soudi added, in a message heard by the thousands of clustering men, many in the black clothes of Mr. Sadr’s Shiite militia.

Mr. Sadr cut short a trip abroad to return because of the bombing and put out his own commands for calm.

Ayatollah Sistani, the single most influential Shiite leader in Iraq, allowed himself to be filmed meeting with other top Shiite religious leaders after the bombing. The rare appearance, televised without sound, underscored the gravity with which Iraq’s leaders took the bombing.