BAGHDAD -- Tensions between Iraq's major Shiite Muslim factions erupted into violence yesterday as Iraqi security forces launched a major crackdown against militiamen in the southern oil hub of Basra.

The fighting, which Iraqi officials said killed at least 35 people and injured 100, was the most serious sign yet that a cease-fire credited with helping improve security nationwide might be unraveling as sections of the Shiite Muslim majority jockey for position ahead of provincial elections.

Heavy explosions and machine-gun fire rocked Basra, where rival political factions, their allied militias and criminal gangs are vying for control of oil exports that generate most of Iraq's government revenue.

Representatives of the radical Shiite cleric Muqtada al-Sadr announced nationwide protests against what they said was the targeting of his followers. The unrest quickly spread to Baghdad, where U.S. and Iraqi officials said armed Shiite militiamen descended into the streets in some neighborhoods and fired a barrage of rockets or mortar rounds at the U.S.-fortified Green Zone.

It was the second time this week that the protected enclave, which houses the U.S. Embassy and Iraqi government offices, took fire from Shiite sections of Baghdad. An auditor for the federal Office of the Special Inspector General for Iraq Reconstruction died Monday of wounds sustained in an attack on Sunday, a spokeswoman said.

A U.S. soldier was killed yesterday when a patrol was attacked with mortar fire in west Baghdad, a military spokesman said. The death lifted to 4,001 the number of U.S. personnel killed since the start of the Iraq war in 2003, according to the independent Web site icasualties.org.

Clashes were also reported between members of al-Sadr's Mahdi Army militia, who waged two major uprisings in 2004, and the Iraqi security forces in the southern cities of Kut and Hillah.

Analysts warned that the fighting could spread as the Shiite factions used their influence within Iraq's security forces to weaken their rivals ahead of the Oct. 1 polls. Yesterday's violence "looks like a preview of what will happen as we approach provincial elections in the fall," said Joost Hiltermann, Middle East director for the International Crisis Group.

Iraqi authorities imposed overnight curfews in many southern cities in a bid to contain the bloodshed.

Basra residents have complained of deteriorating security since British forces handed responsibility for protecting Iraq's second-largest city to the provincial government in December. Stability in the city is crucial if the government is going to attract the kind of investment it needs to expand oil exports and grow the economy. Prime Minister Nouri al-Maliki flew to Basra Monday to oversee the crackdown, which he said would clear the city of "criminals, terrorist forces and outlaws."

Many residents cowered in their homes, listening to explosions after the operation began shortly after midnight.

"Until now, we have been living in continuous anxiety and fear," said Ali Falih, a retired Basra resident reached by telephone at his home. "I expect that this security operation will take long time and maybe last for months, because the Mahdi Army's numbers are not few."

British forces based on the city's outskirts said they were conducting aerial surveillance for the Iraqi government, but did not participate in yesterday's fighting.

Al-Sadr ordered his militia to stand down in August after clashes with a rival militia in the Shiite holy city of Karbala that killed more than 50 people. U.S. commanders say the order played a key part in a 60 percent drop in attacks since a troop buildup President Bush ordered last year reached its height in June.

Al-Sadr renewed the cease-fire for another six months in February, but told his militiamen they could defend themselves against attacks. Clashes have broken out during raids by U.S. and Iraqi security forces in Baghdad, Kut, Diwaniya and other areas in the southern Shiite heartland.

The U.S. military says it is targeting rogue elements of al-Sadr's militia who continue to attack its forces, allegedly with Iranian backing, though Tehran denies the charges. Al-Sadr loyalists accuse his Shiite rivals in the Supreme Iraqi Islamic Council and al-Maliki's party, Islamic Dawa, of using the Iraqi army and police to round up the cleric's followers ahead of the elections.

Al-Sadr's followers hold as many seats in the national parliament as the Supreme Council, and the cleric provided key backing when al-Maliki was named prime minister. But large parts of al-Sadr's movement boycotted the last provincial polls in 2005, handing the Supreme Council and Dawa control of most of the south. A smaller Shiite party called Fadhile holds the position of Basra provincial governor.

Hiltermann said al-Maliki's government was right to try to assert control in Basra. But he said he was not "overly optimistic" that it would succeed - "partly because the central government and its security forces aren't very strong, and partly because this is really a fairly transparent partisan effort by the Supreme Council dressed in government uniforms to fight the Sadrists and Fadhile." Al-Sadr has used the cease-fire to rein in rogue elements he said were disregarding his orders. Sheik Harith Athari, who heads al-Sadr's office in Basra, said the movement had initially agreed to support the government's crackdown, provided that it targeted "outlaws." But he accused the Iraqi security forces yesterday of indiscriminate arrests.

"It is the right of everyone to defend himself if he is attacked without justification," Athari said.

Another official in the Basra office, speaking on condition of anonymity, warned that "The Sadr current (or movement) is threatening to set fire to the oil wells in Basra if the Iraqi military continues its security plan." Al-Sadr's representatives walked out of parliament yesterday to protest the crackdown in Basra.

Alexandra Zavis writes for the Los Angeles Times.