A J A F, Iraq, Aug. 29— A massive car bomb exploded at the Imam Ali mosque during prayers today in this holy city, killing one of Iraq's most important Shiite clerics and at least three other people, witnesses said.
Al-Jazeera television said more people than 20 died. Dozens were injured in the blast, which dug a crater about 3½ feet deep in the street in front of the mosque and destroyed nearby shops, where people pulled the dead and injured from the rubble.
Among the dead was Ayatollah Mohammed Baqir al-Hakim, who had just delivered a sermon calling for Iraqi unity at the shrine, the holiest in Iraq.
Shiites in Iraq are embroiled in a generational power struggle, but there was no evidence the bombing was the work of the younger Shiite faction, which has its strongest support in Baghdad's Sadr City slum.
Even so, both the al-Hakim supporters and a prominent figure in the U.S.-backed government blamed Saddam Hussein loyalists.
Murthada Saeed al-Hakim, al-Hakim's nephew who spoke to the family in Najaf, told The Associated Press the cleric had been killed.
"I saw al-Hakim walk out of the shrine after his sermon and moments later, there was a massive explosion. There were many dead bodies," said Abdul Amir Jassem, a 40-year-old merchant who was in the mosque and said the cleric had prayed for Iraqi unity.
Ayatollah al-Hakim was the spiritual leader of the Supreme Council for Islamic Revolution in Iraq and had divided his time since the end of the war between Tehran and Najaf, the holiest Shiite Muslim city in Iraq.
Mohsen Hakim, another of the cleric's nephews and a spokesman for the Supreme Council of the Islamic Revolution of Iraq, said in Tehran that Saddam loyalists were the prime suspects behind the killing, and he called on the U.S. occupation forces to identify the murderers.
Ahmad Chalabi, the head of the Iraqi National Congress and a Governing Council member, blamed U.S. forces for not keeping the region secure. Speaking on Al-Jazeera, also said Saddam supporters were behind it, saying they were trying to create sectarian discord in the country.
No coalition troops were in the area of the mosque out of respect for the holy site, Pentagon spokesman Lt. Col. Jim Cassella said in Washington.
The top U.S. civilian official in Iraq, L. Paul Bremer, denounced the bombing, saying it demonstrated that "the enemies of the new Iraq will stop at nothing."
"Again, they have killed innocent Iraqis. Again, they have violated one of Islam's most sacred places. Again, by their heinous action, they have shown the evil face of terrorism," Bremer said in a statement.
There has been considerable unrest among the religious factions in Najaf.
The al-Hakims are one of the most influential families in the Iraq's Shiite community. The ayatollah's brother, Abdel-Aziz al-Hakim, is a member of the Governing Council and was leader of the armed wing of the Supreme Council for Islamic Revolution in Iraq, headquartered in Iran before the war.
Younger Shiites have been fighting for power with the more traditional Shiite Muslims in the city and region, trying to grab control from the al-Hakim family.
Sheik Muqtada al-Sadr, who is not yet 30, and his young followers have sought tirelessly to replace more traditional factions as the voice of Iraq's Shiite majority, portraying themselves as the ones doing the most to redress decades of suppression by Sunni Muslims under the Saddam's rule.
"The killing appears to have sought to deny Shiite Muslims an effective role in Iraq's future at a time when Iraq is gradually preparing for elections," said Iranian political analyst Morad Veisi in Tehran.
He said the killers sought to sow seeds of discord between Shiite and Sunni Muslims and showed the United States is "incapable of providing adequate security in Iraq."
The blast occurred a week after a bomb exploded at the house of another of Iraqi's most important Shiite clerics, killing three guards and injuring 10 others, including family members. The gas cylinder was placed along the outside wall of the home of Mohammed Saeed al-Hakim in Najaf. It exploded just after noon prayers Aug. 24. Mohammed Saeed al-Hakim is related to the ayatollah who was killed today.
A day after Saddam's ouster, a mob in Najaf hacked to death a Shiite cleric who had returned from exile. Abdul Majid al-Khoei was killed when a meeting called to reconcile rival Shiite groups erupted into a melee.