Followers of radical anti-U.S. cleric Muqtada al-Sadr tear up an American flag in the holy city of Najaf, Iraq, on the fourth anniversary of the fall of Baghdad. Thousands of Iraqis draped themselves in Iraqi flags and marched Monday through the streets of Shiite holy cities Najaf and Kufa in a rally that was called for by al-Sadr.

Najaf, Iraq ? Flying banners that said “No, no to the occupation,” tens of thousands of followers of the anti-American Shiite Muslim cleric Muqtada al-Sadr staged a peaceful rally Monday in one of Iraq’s holiest cities, where they burned American and Israeli flags and called for U.S. troops to leave Iraq.

But there were ominous signs afterward that al-Sadr’s enormous Mahdi Army militia may be preparing for renewed violence in Baghdad.

Monday evening, as protesters returned in trucks and buses to Baghdad’s sprawling Shiite slum of Sadr City, a McClatchy Newspapers reporter saw men in several buses carrying pistols and AK-47s, a violation of new security laws. One man who identified himself as a Mahdi Army member bragged that weapons were being taken from Najaf to Baghdad hidden in truck beds.

The protest in Najaf, which al-Sadr had called as a gesture of national unity, took place under heavy Iraqi security on the fourth anniversary of the fall of Saddam Hussein’s regime. Protesters waving red, white and black Iraqi flags marched about five miles from a mosque in the town of Kufa to Najaf, 100 miles south of Baghdad.

Although the Mahdi Army is blamed for the widespread kidnapping and killing of Sunni Muslims, over the weekend al-Sadr urged Iraqi security forces and all Iraqi people to band together against U.S.-led coalition forces.

The rhetoric at the rally was menacing at times. “The occupation and the people connected to it will vanish,” the demonstration’s organizers said in a statement, “and Iraq will stay for Iraqis and the country for its sons.”

Much of the country was quiet Monday after a last-minute government decree made it a national holiday. Vehicles were barred from the streets of Baghdad for 24 hours, until 5 a.m. today.