Police said the attack involved explosives and brought down the two minarets that had flanked the Al-Askari (Golden) Mosque in Samarra.
In February 2006, a bomb attack destroyed the mosque's huge golden dome, sparking a wave of sectarian violence in the country that left hundreds dead and brought the country to the brink of civil war. The bombing was blamed on Sunni militants believed to be linked to Al-Qaeda.
Fears Of Sectarian Violence
Thus there are fears today that this latest desecration of the famous religious shrine could further escalate the cycle of violence between Shi'a and Sunnis.
Prime Minister Nuri al-Maliki immediately met with the U.S. commander in Iraq, General David Petraeus, to ask that U.S. reinforcements be sent into Samarra to help head off violence in the city.
In a televised address, al-Maliki said security would be stepped up. "I have instructed the security forces to take more effective precautions to protect all holy sites and mosques against troublemakers seeking to incite strife," he said. "We have also set up a commission of inquiry that has detained all those assigned to guard the holy shrine as part of the investigation into this crime. Our judgement will be harsh on those proven to be involved in this crime."
State television later reported that an indefinite curfew would be imposed in Baghdad, starting at 3 p.m. local time (1100 UTC). The Iraqi capital is already under a nightly curfew between 11 p.m. and 6 a.m.
Shi'ite Leaders Call For Restraint
In his televised address, al-Maliki called for calm.
"I call upon all citizens and believers and ask religious leaders and authorities to impress upon the public the need for restraint and to display caution and wisdom in order to frustrate those fishing in troubled waters, those who want to take advantage of this crime for other political ends," he said. "I also warn those fishing in stagnant waters not to harm the government of national unity and its legitimacy."
Radical Shi'ite cleric Muqtada al-Sadr, meanwhile, has called for three days of mourning and for peaceful demonstrations to mark the destruction of the minarets.
Through a spokesman, revered cleric Grand Ayatollah Ali al-Sistani, Iraq's top Shi'ite religious leader, called for restraint.
Al-Maliki made a simlar call. "I call upon all citizens and believers and ask religious leaders and authorities to impress upon the public the need for restraint and to display caution and wisdom in order to frustrate those fishing in troubled waters, those who want to take advantage of this crime for other political ends. I also warn those fishing in stagnant waters not to harm the government of national unity and its legitimacy," he said.
Although Samarra is mostly Sunni, the Golden Mosque is one of the four major Shi'ite shrines in Iraq.
The Al-Askari Mosque is on of Shi'ite Islam's holiest sites. Two of the 12 Shi'ite imams -- Imam Ali al-Hadi, who died in 868 A.D., and his son, Imam Hasan al-Askari, who died in 874 A.D. -- are buried at the mosque. The complex also contains the shrine of the 12th imam, Muhammad al-Mahdi, who is said to have gone into hiding through a cellar in the complex in 878, and is expected to return on Judgment Day.