Baghdad, Iraq ? Grand Ayatollah Ali Sistani, Iraq’s supreme Shiite religious leader, demanded Tuesday that all armed forces leave the holy city of Najaf and called on fellow Shiites not to join in a bloody uprising there against U.S. forces. It was his first public effort to end a weeks-old rebellion mounted by the radical cleric Muqtada al-Sadr.

Sistani was apparently responding to a call to arms issued earlier in the day by al-Sadr, whose Mahdi Army militia has largely controlled Najaf for weeks. Sistani’s words are often heeded by Shiites, although his call Tuesday was not a religious order, known as a fatwa.

Earlier in the day al-Sadr invited all Iraqis to come to the southern city and support his uprising, which U.S. troops are struggling to contain. The revolt is one of several serious security issues that U.S. officials face before the scheduled transfer of limited authority to an Iraqi-led government on June 30.

U.S. military options are constricted in part because Najaf is home to one of the most revered sites in Shiite Islam — the Shrine of Imam Ali — and a vast graveyard that is the most favored burial spot among Shiites due to for its proximity to the mosque.

For the past month, U.S. officials have been hoping Sistani would stand up to al-Sadr, who has little authority beyond what he commands through his thousands-strong militia. Al-Sadr has previously said he would follow a request from Sistani to withdraw from the city, but his rhetoric has growing increasingly militant the longer he has kept U.S. forces from rolling into the city in force.

“So rise up my beloved people,” al-Sadr said in the statement issued by his office in Najaf.

He called on “the people of great Iraq to express your opinion” in Najaf “as a reply to the serial violations in order to be the best people for the best sacred shrines.”

Sistani has traditionally shied away from political matters. His boldest such overture came last November, when he called for direct elections to establish the post-occupation government, rather than a caucus system favored by U.S. officials.

Tuesday’s formal statement, a rare personal message to the public by a man who usually communicates indirectly through aides, came a day after his offices in Najaf were fired on, by some accounts by al-Sadr’s men.

The conflicting statements appear to open the way for a test of wills between two clerics with vastly different views of Islam’s role in the political future of Iraq. Shiites account for 60 percent of Iraq’s population, and after years of suffering under former president Saddam Hussein’s Sunni-led government are clamoring for a large political stake in the post-occupation government.

The Shiite community was once largely receptive to the U.S. invasion. Its current division is not only complicating U.S. efforts to establish a broadly acceptable interim government but raises the specter of violence between armed militias attached to rival Shiite parties.