Shiite Muslim militias clashed with Iraqi and foreign forces Friday in several cities, and officials said the fighting, which began Thursday, has killed at least 300 militiamen, three U.S. troops and uncounted Iraqi civilians.

Spurred by impassioned calls to arms at Friday prayer services in Baghdad and other cities, masked militia members loyal to the firebrand Shiite cleric Moqtada Sadr, armed with rifles and rocket launchers, roamed freely through Najaf, 90 miles south of Baghdad, and Sadr City, a sprawling Shiite slum in the capital.

Spokesmen for Sadr said Friday that the radical cleric, who has marshaled thousands of rebellious young Shiites, wanted to restore a truce established in early June. That cease-fire followed two months of heavy fighting between his militia and U.S. and allied forces. Mahmoud Soudani, Sadr's spokesman in Baghdad, said: "We have no objections to entering negotiations to solve this crisis. . . . We want a resumption of the truce."

But other Shiite clerics openly called on their followers to kill Americans and join a nationwide uprising against Iraqi authorities, accusing coalition forces of attacking sacred Shiite sites and occupying Iraq.

"The Americans have attacked Najaf and the shrine of Ali. I don't want to hear people say this is wrong, I want them to come here and fight," Hazim Araaji, a local sheik, thundered in a sermon in the central mosque in Kadhimiya, a large Shiite district in Baghdad. "Our god says you must prepare yourself to fight the occupiers. . . . You do not need our permission to fight the Americans. You should do it directly."

As the violence spread, a potential voice of reason among Shiite leaders was absent. Grand Ayatollah Ali Sistani, 73, an influential moderate cleric, was reported to have left Iraq on Friday to seek medical treatment in London for a heart condition.

The most intense conflict was in Najaf, the site of the tomb of Imam Ali, one of Shiism's most revered figures. During 36 hours of sustained fighting, U.S. Marines battled militiamen from Sadr's Mahdi Army, while U.S. helicopters, F-16s and AC-130 gunships circled the city.

Most of the fighting took place in the area of Najaf's vast Shiite cemetery, which had been off-limits to U.S. forces under the June cease-fire agreement. U.S. military officials said hundreds of militia members had taken cover and hidden weapons in the graveyard, which is near the shrine.

"Najaf is being subjected to . . . total destruction," said Ahmed Shaibany, an aide to Sadr in Najaf. "We call on the Islamic world, and on the entire civilized world, to intervene to save the city."

There were conflicting reports of casualties in Najaf. Iraqi officials said at least 400 people had been killed and more than 1,000 militiamen had been arrested. Officers from the 11th Marine Expeditionary Unit based outside Najaf said 300 militia fighters and two unidentified Marines had been killed, but they gave no estimate of casualties among Iraqi security forces or civilians. A U.S. soldier was also killed in the fighting, the Associated Press reported.

Lt. Col. Gary Johnston, the operations officer for the unit, said Marine forces had not broken the truce but had responded late Thursday to a request for help from overwhelmed Iraqi security forces after militia fighters began massing in the cemetery and repeatedly attacking the city. Iraqi officials said the fighting began when militia forces attacked a police station Thursday morning.

"We received heavy fire and were attacked, and we attacked back," Johnston said. He also stressed that U.S. troops were not pursuing Sadr, saying: "Our mission is not focused on any particular individual, it is to support the governor of Najaf. We are not at war with [Sadr]; we are here to support the Iraqi forces and the Iraqi people."

In Sadr City, Iraqi health officials said at least 19 people had been killed and 100 injured in two days of clashes. Residents said U.S. forces had surrounded the community with tanks and had fired from helicopters but had not entered on the ground.

In the southern city of Nasiriyah, officials said militia fighters attacked Italian troops with automatic weapons during fighting that began Thursday and lasted until dawn Friday. Iraqi officials said Iraqi and coalition forces were in control of key sectors of the city by midday, although Sadr's forces were moving freely in one area.

There were also reports of more limited clashes in the southern city of Basra, near the Persian Gulf, where British troops are based. British military officials said their garrison was hit by mortar fire early Friday, and witnesses said sporadic street fighting with automatic weapons continued into the evening. No serious casualties were reported.

In separate violence, officials reported clashes between U.S. troops and Sunni Muslim insurgents in the town of Samarra, about 70 miles north of Baghdad. Hospital officials said at least two people were killed.

In the volatile Sunni region of north-central Iraq, the governor of Anbar province abruptly resigned Friday, apparently under pressure from insurgents who had set his house on fire and kidnapped his three sons one week ago in the city of Ramadi, west of Baghdad.

The governor, Abdul Karim Rawi, appeared on Arabic TV, looking grim and subdued and read a statement saying he "repented" having acted in collaboration with "the infidels, the Americans." The video then showed Rawi on the floor of a room, weeping and hugging his sons, who appeared to have been released.

Meanwhile, Lebanese officials said Friday that four Lebanese truck drivers had been reported missing in Iraq. They were believed to be the latest victims in a rash of kidnappings in which Islamic militant groups and criminal gangs have abducted drivers as well as other foreigners.

Iraqi officials said Friday that they were confident they could contain the violence in the south, and they referred to the Shiite militiamen as terrorists and criminals. Georges Sada, a spokesman for Iraq's interim prime minister, Ayad Allawi, said the government "will fight them and will not allow their criminal actions in the various cities, irrespective of who they are or how big they are."

But other comments made Thursday by Interior Minister Falah Naqib appeared to have further inflamed the Shiite populace and were invoked in rousing sermons. Falah advised Sadr and his troops to stop "killing yourselves" and said it was "good news" that several militiamen had been killed.

Several Shiite leaders asserted that coalition forces had attacked sacred sites without provocation and that the dome of the Imam Ali shrine had been damaged. But Iraqi and U.S. officials said Sadr's forces had instigated the violence by attacking the police station.

"We did not want to escalate the situation . . . but the actions of the American troops have enraged the sons of these cities," said Soudani, Sadr's spokesman in Baghdad.

The previous flare-up of Shiite violence, in April, led to two months of intense fighting and was triggered when U.S. authorities issued an arrest warrant for Sadr and shut down his movement's newspaper in March.

Another object of Shiite anger was the Najaf governor, Adnan Zurfi, a close U.S. ally. In speeches and sermons, Shiite clerics have called him a "lunatic" and a "lackey" of U.S. forces.

Soudani said Sadr's movement held U.S. troops and Zurfi "totally accountable for the state the crisis has reached to now." Late Friday, Zurfi issued an ultimatum to Sadr's forces, giving them 24 hours to leave the city.

Special correspondents Omar Fekeiki, Bassam Sebti and Luma Mousawi contributed to this report.

Shiite militiamen dash across a street during fighting in the Iraq city of Najaf, where U.S. Marines battled the Mahdi Army militia. An Iraqi asks to pass U.S. soldiers searching a Baghdad hospital for fighters from cleric Moqtada Sadr's Mahdi Army. During a second day of clashes, Iraqi Shiite militiamen patrol the streets of the holy city of Najaf, where clerics claim the U.S. has damaged Shiite shrines.