Eyewitnesses say the car bomb exploded after Friday prayers outside the tomb of Ali, in Najaf, one of Shiite Islam's holiest sites.

Damaged Imam Ali Mosque

The bombing follows an attack last Sunday against a relative of Ayatollah al-Hakim, in which three of his guards were killed. Rival groups within the Shiite leadership are being blamed for the violence. The cleric, who was persecuted under the regime of Saddam Hussein, had also been criticized by some for allowing followers to work with the U.S.-led coalition authority.

The head of the authority, Paul Bremer, issued a statement denouncing the bombing as the evil face of terrorism, and pledging full cooperation with the Iraqi police in the investigation. He also expressed sympathy to the families of the victims.

U.S. military officials say an American soldier was killed Friday by rocket propelled grenades in an attack near the town of Baquba, 65 kilometers north of Baghdad. In addition, two U.S. soldiers were wounded in Fallujah, 50 kilometers west of the capital, when their convoy was hit by a rocket propelled grenade.

Meanwhile, coalition forces announced the seizure of surface-to-air missiles, mortar rounds and grenades in several raids in central Iraq. Twenty-four people were arrested during this operation.

Coalition leaders say they are working to bolster security in Iraq following terrorist attacks on U.N. headquarters and the Jordanian embassy in Baghdad and amid continuing attacks on coalition forces in which more than 60 soldiers have died since the end of major combat was declared in May.

The attacks, combined with a resurgence of violent crime in Baghdad, have led some humanitarian agencies to withdraw foreign staff members at least temporarily.

The spokeswoman for the United Nations Humanitarian Coordinator, Veronique Taveaux, says she understands the reaction of some non-governmental organizations, or NGO's, because the nature of their work puts them at risk.

"As humanitarian workers, we have to go on [into] the field, we have to go outside," she said. "We cannot stay inside the houses, or premises, or compounds that we have. So if we cannot do that for security reasons, and obviously at the moment we can't do that, I can absolutely understand why some NGO's are pulling out."