The protest was in some ways another challenge to the Shiite clerical hierarchy, showing that in the new Iraq, a violent young upstart like Mr. Sadr can command the masses right in the backyard of venerable clerics like Ayatollah Sistani. Mr. Sadr has increasingly tapped into a powerful desire among Shiites to stand up forcefully to both the American presence and militant Sunnis, and to ignore calls for moderation from older clerics.

Lt. Col. Christopher Garver, an American military spokesman in Baghdad, said that American officers had helped officials in Najaf plan security for the event, but that the Iraqis had taken the lead.

Colonel Garver and other American officials tried to put the best possible light on the event, despite the fiery words. “We say that we’re here to support democracy,” he said. “We say that free speech and freedom of assembly are part of that. While we don’t necessarily agree with the message, we agree with their right to say it.”

The protest unfolded as heavy fighting continued in parts of Diwaniya, a southern city where American and Iraqi forces have been battling cells of the Mahdi Army since Friday. Mr. Sadr issued a statement on Sunday calling for the Mahdi militiamen and the Iraqi forces there to stop fighting each other, but those words went unheeded. Gun battles broke out on Monday, and an American officer said at a news conference that at least one American soldier had been killed and one wounded in four days of clashes.

That fighting and the protest in Najaf, as well as Mr. Sadr’s mysterious absence, raise questions about how much control he actually maintains over his militia. Mr. Sadr is obviously still able to order huge numbers of people into the streets, but there has been talk that branches of his militia have split off and now operate independently. In Baghdad, some Mahdi Army cells have refrained in the last two months from attacking Americans and carrying out killings of Sunni Arabs, supposedly on orders from Mr. Sadr, but bodies of Sunnis have begun reappearing in some neighborhoods in recent weeks.

The protest in Najaf was made up mostly of young men, many of whom drove down from the sprawling Sadr City section of Baghdad, some 100 miles north, the previous night. They gathered Monday morning in the town of Kufa, where Mr. Sadr has his main mosque, and walked a few miles to Sadrain Square in Najaf. Protesters stomped on American flags and burned them. “No, no America; leave, leave occupier,” they chanted. At Sadrain Square, the protesters listened to a statement read over loudspeakers that was attributed to Mr. Sadr.