The heaviest fighting occurred mainly in Najaf, a Shiite holy city 100 miles south of Baghdad that is a stronghold for al-Sadr. A Marine helicopter was shot down there, but the crew members were evacuated safely, the U.S. military reported.

Baghdad, even in the Shiite slum neighborhood of Sadr City, appeared to be mostly quiet until 11:15 p.m., when three large explosions, probably from mortars, rocked the city's center. Small-arms fire followed.

One Marine and several insurgents were killed in Najaf, where Marines fought alongside Iraqi policemen and National Guard troops. At least a dozen more soldiers and dozens of insurgents were wounded in both Baghdad and Najaf, though exact casualty counts were unavailable Thursday night.

Near midnight, al-Sadr offered a tentative cease-fire, saying his guerrillas would stop fighting if U.S. soldiers did the same, according to a spokesman for the group. The offer would renew a 2-month-old truce between al-Sadr and the U.S. military.

Each side blamed the other for the apparent breakdown of the cease-fire, which comes less than two weeks before a national political conference that al-Sadr has said he will not attend.

While more confined than the widespread fighting in April and May, Thursday's attacks represented the most serious challenge yet to the interim Iraqi government, whose prime minister, Ayad Allawi, has struggled to assert his authority since being appointed in June. Unlike moderate Shiite political leaders like Allawi, al-Sadr fiercely opposes the continuing American presence here and has tried twice since October to revolt against it.

Allawi, who has been traveling outside Iraq for most of the last 10 days, is anxious to show his independence from the United States and to prove that Iraqi security forces can stop the growing violence here. But Thursday's clashes showed again that only U.S. troops have the firepower to contain al-Sadr's self-designated Mahdi Army, a well-armed militia that has fighters across the southern half of Iraq.

During the afternoon, U.S. jets swooped over Baghdad and appeared to drop several bombs on Sadr City, a giant Shiite slum in Baghdad. A military spokesman confirmed that an F-15 fighter had dropped at least one bomb.

Baghdad, which has recently been racked by a spate of kidnappings of both Iraqis and Westerners, was very tense on Thursday as word of al-Sadr's call for an uprising spread. There are no reliable estimates of the exact size of the Mahdi Army, but in the past al-Sadr has shown he could bring thousands of armed men into the streets.

In Sadr City, masked Mahdi Army guerrillas controlled intersections and checked cars. Iraqi police officers and U.S. soldiers remained outside the area for most of the day, and an Iraqi employee of The Times who entered the area twice during the afternoon said he had seen no signs of fighting.

But the Iraqi police reported several gunbattles in Sadr City during the late afternoon, and a U.S. military spokesman told The Associated Press that seven soldiers had been wounded in two skirmishes in the area.

In Basra, a mostly Shiite city in the southeastern corner of Iraq, a spokesman for al-Sadr said the Mahdi Army had 1,000 guerrillas ready to fight, according to Reuters.

The fiercest fighting occurred in and around Najaf, where at least seven insurgents were killed and at least 22 wounded, according to a military statement. The shrine of Imam Ali, a mosque in the center of Najaf that is among the holiest sites to Shiites, was slightly damaged in the fighting, The AP reported. In general, U.S. troops have tried to avoid fighting around the shrine and other Shiite holy sites.

The truce between al-Sadr and the military has been unraveling for days.

Falah al-Nakib, the Iraqi interior minister, blamed al-Sadr's forces for the fighting.

''They attacked Iraqi police and we must respond,'' Nakib said. ''We have the thugs isolated. Our police forces, supported by the multinational force, are doing their job.''