BAGHDAD, 13 August 2004 — Iraqi cleric Moqtada Sadr ended an uprising he led earlier this year with a truce, but this time he seems poised for a final showdown.

US Marines launched an offensive yesterday to sweep his Mehdi Army militia out of the southern city of Najaf, aiming to snuff out a weeklong rebellion raging across southern Iraq.

Holed up in Najaf, Sadr has staked his credibility on fighting the American onslaught to the end — sheltering his rag-tag fighters in the city’s Imam Ali Mosque, one of the holiest shrines of Shiites, and a nearby cemetery.

“The Mehdi Army and I will keep resisting. I will stay in holy Najaf and will never leave,” Sadr said this week. “I will stay here until my last drop of blood.” The uprising has posed the biggest challenge to the interim government of Prime Minister Iyad Allawi since it assumed powers from US occupiers on June 28, killing perhaps hundreds of militiamen and causing heavy civilian casualties.

Sadr’s plump, bearded features have become one of the most recognizable faces of resistance to Iraqi authorities, even though his Mehdi Army militia is just one of many groups bent on undermining the interim government’s authority.

But Sadr — who faces murder charges in connection with the killing of a rival cleric — enjoys far from universal support among Iraq’s majority Shiites, relying on his status as a scion of a Shiite clerical dynasty for much of his authority.

Within days of US troops ousting Saddam Hussein in 2003, Sadr, aged about 30, was seeking to rouse Iraq’s long-oppressed Shiite majority against the occupiers and their Iraqi allies. Providing a mouthpiece and the muscle of his Mehdi Army militia for the impatient poor, Sadr became a familiar sight on posters held up by chanting supporters.

Capitalizing on frustration among Iraqis over insecurity and poverty since the US-led invasion, Sadr won followers by supplying health care and other services. He also built up an armed force thousands strong in towns from Baghdad to Basra, wherever Shiites were in the majority.

Baghdad’s Shiite slum had its name changed by popular consent from Saddam City to Sadr City, becoming his power base. When the former US-led administration in Iraq closed down his newspaper and arrested a key aide in early April, the result was not capitulation but a widespread uprising.

Even Iraq’s top Shiite cleric, Ayatollah Ali Al-Sistani, has trodden warily around Sadr, refusing to condemn his actions despite the general upset they caused among Shiite elders. Sistani conspicuously flew to London last week for medical treatment just as the fighting started in Najaf.

Sadr derives his own authority from his father, Ayatollah Mohammed Sadeq Al-Sadr, who paid with his life for defying Saddam from within Iraq. He and two elder sons were murdered in 1999, probably by the dictator’s agents.

Sadr’s uncle, Mohammed Baqer, was also killed by Saddam in 1980 after he called for an Iranian-styled Islamic republic in Iraq. Sadr has joined a power struggle among Shiite factions in Najaf and has links to clerics in neighboring, Ruled-ruled Iran.

Sadr denies involvement in the murder in Najaf in April 2003 of Abdul Majid Al-Khoei, son of another late ayatollah. Khoei, who had close British and US ties, was hacked to death at the Imam Ali Mosque days after returning from exile.