Accompanied by police escorts and wailing relatives, coffin-laden vans drove from Baghdad to Najaf in an exception to the open-ended citywide curfew that was imposed after the blasts. Outraged mourners, most of them armed with machine guns, said al-Maliki's calls for restraint in the aftermath of the blasts no longer held any appeal.

Women screamed and tore at their clothing. Men shouted for "revenge against the Americans and the terrorists."

Hussam Moussa, 30, who lost five members of his family in the bombings, placed equal blame on those who staged the attacks, presumably Sunni insurgents, and those who failed to prevent them: U.S. and Iraqi security forces.

"We will respond to this cowardly act. We won't spare a single Sunni in Baghdad," Moussa said. "They will see a sectarian war. It's either us or our enemies and, by God, they will see wonders. We will kill infants, women and the elderly. Everything will burn."

Describing Bush as "the killer of Iraqi people," politicians aligned with al-Sadr repeated their demand for the withdrawal of U.S. troops and said Iraqi officials should sever all ties with the United States. Al-Sadr's allies are the backbone of al-Maliki's Cabinet, and even a temporary suspension of their participation would cripple, if not topple, Iraq's fragile U.S.-backed government.

"They are telling the ordinary people that if the American forces withdraw from Iraq, this will provoke more violence. We say, since the minute they stepped on this ground, chaos and instability have spread throughout the country," said Saleh Hassan al-Agili, one of 30 legislators from al-Sadr's parliamentary bloc. "We reiterate that the departure of the occupying forces will restore stability, security and the brotherhood of the Iraqi people."

Residents in some Sunni areas of Baghdad said they saw U.S. patrols in their areas Friday, but a U.S. military spokesman would confirm only troop movements to "enhance existing security arrangements." An American attack helicopter destroyed rocket launchers in Sadr City after Shiite militants there fired six times into a Sunni neighborhood, according to a U.S. military statement. No casualties were reported. Spokesmen for al-Sadr denied reports that Shiites executed dozens of Sunni men, but an Iraqi satellite news channel late Friday night quoted police officials saying that residents had taken cell-phone pictures of the killings.

Al-Sadr issued a challenge to his onetime ally and current nemesis, leading Sunni cleric Hareth al-Dhari, during prayers Friday in the southern Shiite city of Kufa.

"I demand Sheik Hareth al-Dhari issue a fatwa (a religious decree) prohibiting the killing of Shiites, so as to spare Muslim blood," al-Sadr told about 5,000 followers.

He also called on his Sunni counterpart to ban his supporters from joining al-Qaida or other terrorist groups, and to participate in rebuilding a Shiite shrine in Samarra that was heavily damaged in a February bombing that unleashed the sectarian bloodletting.

In exchange for al-Dhari's agreement, al-Sadr said, he'd condemn any aggression against the Sunni cleric, who leads the militant Association of Muslim Scholars. Al-Dhari has been out of the country for several weeks and didn't respond immediately to al-Sadr's challenge.

Al-Sadr also beseeched rival Shiite parties to put aside their political differences and unite in a call for calm. Otherwise, he implied, the central Shiite religious authority, known as the hawza, could become irrelevant.

"Why has the devil made his way between us? This will serve only the colonizers and will harm the hawza," al-Sadr said. "Here is my hand — I put it forward in reconciliation. Will there be a hand reaching out for mine?"