Four holes, each approximately 12 inches long and 8 inches wide, could be seen on the golden dome of the Imam Ali mosque, burial place of Imam Ali Ibn Abu Talib, the Prophet Muhammad's cousin and son-in-law and the Shiites' most revered saint.

The mosque, in the middle of Najaf, is about 100 miles south of Baghdad on a high desert plateau overlooking the world's largest cemetery.

Militia members blamed the Americans for the damage to the mosque, but Brig. Gen. Mark Kimmitt, the chief U.S. military spokesman in Iraq, said al-Sadr's men were probably responsible: "I can just tell you by the looks of where we were firing and where Muqtada's militia was firing, I would put my money that Muqtada caused it."

During the crackdown on al-Sadr's al-Mahdi Army, U.S. forces have been careful to avoid damaging shrines for fear of enraging Iraq's Shiite majority. They have attacked mosques where insurgents have set up fighting positions.

At a press conference in Baghdad, Kimmitt pointed to a map of Najaf and said a U.S. convoy might have been fired on from the cemetery as it moved near the shrine. If so, those rounds could have hit the shrine, he said.

Kimmitt accused the militia of using religious sites "much like human shields." He said American forces had not initiated the fighting but were responding to attacks by al-Sadr's gunmen.

That did little to assuage the anger of many Shiites in Najaf. By early evening, thousands gathered around the Imam Ali shrine to inspect the damage. Some shook their heads in disbelief. Others mumbled prayers.

"The Americans had better leave Iraq after this," said Jassim Mohammed. Another man, Abu Zahraa al-Daraji, added: "The Americans have crossed a red line."

Al-Sadr's aides called on their followers to rise up against the coalition. His representative in Nasiriyah, Sheik Aws al-Khafaji, threatened attacks on coalition forces there, most of whom are Italians.

After his threat, armed men attacked coalition headquarters in Nasiriyah, about 200 miles southeast of Baghdad. They fired at least five rocket-propelled grenades within a half hour as Italian troops and Filipino security guards fought back.

About 10 coalition staffers, including Italians, Americans and Britons along with 10 drivers and security guards were trapped in the building along with four Italian journalists, coalition officials said.

"It's an inferno," Maria Cuffaro, a journalist for Italy's state-run RAI network, said during a brief live report on Italian television late Friday. "We're all OK, if a bit shaken."

Explosions and gunfire also rocked another Shiite holy city, Karbala, as U.S. troops clashed with al-Sadr's militiamen. Shops were closed and residents stayed off the streets.