Samarra, Iraq ? Insurgents posing as police destroyed the golden dome of one of Iraq’s holiest Shiite shrines Wednesday, setting off an unprecedented spasm of sectarian violence. Angry crowds thronged the streets, militiamen attacked Sunni mosques and at least 19 people were killed.

With the gleaming dome of the 1,200-year-old Askariya shrine reduced to rubble, some Shiites lashed out at the United States as partly to blame.

The violence – many of the 90 attacks on Sunni mosques were carried out by Shiite militias – seemed to push Iraq closer to all-out civil war than at any point in the three years since the U.S.-led overthrow of Saddam Hussein.

Many leaders called for calm. “We are facing a major conspiracy that is targeting Iraq’s unity,” said President Jalal Talabani, a Kurd. “We should all stand hand in hand to prevent the danger of a civil war.”

President Bush pledged American help to restore the mosque after the bombing north of Baghdad, which dealt a severe blow to U.S. efforts to keep Iraq from falling deeper into sectarian violence.

“The terrorists in Iraq have again proven that they are enemies of all faiths and of all humanity,” Bush said. “The world must stand united against them, and steadfast behind the people of Iraq.”

Iraqis gather Wednesday at the ruins of a shrine in Samarra. A large explosion Wednesday heavily damaged the golden dome of one of Iraq's most famous Shiite religious shrines. It was the third major attack against Shiite targets in as many days.

British Prime Minister Tony Blair also condemned the bombing and pledged funds toward the shrine’s reconstruction.

U.S. Ambassador Zalmay Khalilzad and the top American commander in Iraq, Gen. George Casey, called the attack a deliberate attempt to foment sectarian strife and warned it was a “critical moment for Iraq.”

No one was reported injured in the bombing of the shrine in Samarra.

But at least 19 people, including three Sunni clerics, were killed in the reprisal attacks that followed, mainly in Baghdad and predominantly Shiite provinces to the south, according to the Iraqi Islamic Party, the country’s largest Sunni political group.

Many of the attacks appeared to have been carried out by Shiite militias that the United States wants to see disbanded.