BAGHDAD, Iraq — 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 may be unraveling as sections of the Shiite Muslim majority jockey for position ahead of upcoming 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 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 American 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 American-fortified Green Zone.

It was the second time this week that the protected enclave, which houses the American 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.

An American 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 American 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 Mr. 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.