Iraqis march during a protest against the U.S. military presence in Sadr City, a Shiite enclave in Baghdad, Iraq. Some 10,000 people protested Friday against the opening of a base for U.S. and Iraqi forces in the enclave.

Baghdad, Iraq ? After weeks of cooperation with a new security plan, radical Shiite cleric Muqtada al-Sadr decried U.S. forces as occupiers Friday and called on his followers to “shout ‘No, No America!'” in a sign of resurgent anger and opposition.

Thousands of Shiites flooded from the mosque where al-Sadr’s statement was read by a preacher at Friday prayers, spilling into the streets of the Sadr City slum to protest the 2-week-old American military presence there. The U.S. military says al-Sadr has gone to Iran.

Officials with al-Sadr’s Mahdi Army did not explain why al-Sadr chose to issue the surprisingly confrontational statement.

American military leaders had credited al-Sadr – who was said to have ordered his Mahdi Army militia to put away its weapons and not confront U.S. and Iraqi troops – for the relatively effortless start of security patrols and raids in the volatile Shiite slum, a no-go zone for U.S. forces until about two weeks ago.

Al-Sadr’s message on the Muslim day of prayer and rest could signal a shift in his willingness to absorb the perceived indignity of the U.S. troop presence and wait out the security plan. Or it could have been nothing more than a reminder to his followers that he was watching carefully and still was their leader.

“The occupiers want to harm this beloved (Sadr City) and tarnish its name by spreading false rumors and allegations that negotiations and cooperation are ongoing between you and them,” the statement said. “I am confident that you will not make concessions to them and will remain above them. Raise your voices in love and brotherhood and unity against your enemy and shout ‘No, No America!'”

Al-Sadr’s office in the holy city of Najaf, 100 miles south of Baghdad, confirmed the statement was genuine.

American military officials said the fiercely anti-American cleric remains in Iran, where he was said to have fled in the days preceding the security operation.

The reference in the al-Sadr statement to “negotiations” may have been intended as a reminder to followers not to go too far in cooperating the Americans.

Sadr City Mayor Rahim al-Darraji, a principal negotiator with American forces, was seriously wounded in a shooting attack on his convoy Thursday.

His negotiation work had created tension in the ranks of Shiite militiamen and some blamed the assault – which also killed two bodyguards – on a militia faction unhappy about cooperation with the U.S. military, a local Mahdi Army commander said Friday.

“This is a faction that enjoys some weight,” the commander said, speaking on condition of anonymity because of the sensitivity of the subject.

One of the dead bodyguards was identified as police Lt. Col. Mohammad Mutashar Al-Freji, a friend of al-Darraji’s who was politically linked to al-Sadr.

Late Friday, the American military reacted cautiously to the al-Sadr statement.

“We have often seen differing political views or differing statements coming out of many of the political organizations here in Iraq, not just the Sadr bloc or al-Sadr’s organization,” U.S. military spokesman Lt. Col. Christopher Garver said. “As we’ve said, we are, if anything, cautiously optimistic, but it’s still very early.”