It was the second major demonstration by Sadr's followers in recent days after the cleric issued a warning on April 9 he would unleash his Mehdi Army militia if U.S. troops were not out of Iraq by December 31. More than 5,000 marched in the streets of Basra, Iraq's southern oil hub, on Thursday.

"I am ready to fight an American again and I am ready to die for Iraq," Haider al-Bahadili, a 33-year-old Mehdi Army member, said at Saturday's demonstration in western Baghdad's Shula district. "Decisions of Moqtada are orders. We must apply them."

The march marked the seventh anniversary of the formation of the Mehdi Army, which fought U.S. forces after the 2003 U.S.-led invasion that toppled Saddam Hussein.

The rally came a day after Admiral Mike Mullen, chairman of the U.S. military's Joint Chiefs of Staff, said during a visit to the capital Iraq's leaders must move quickly if they want U.S. forces to stay beyond year-end.

About 47,000 American troops still advise and train Iraqi forces. Prime Minister Nuri al-Maliki has said his police and army are ready to provide security and foreign troops will no longer be needed after December 31.

Reuters