Thousands of Iraqis have marched through the streets of two Shi'ite holy cities to call for the withdrawal of U.S. troops and to mark the fourth anniversary of the fall of Baghdad.

Men, women and children carrying Iraqi flags and shouting anti-American slogans marched through Najaf and the neighboring city of Kufa  Monday.

Radical Shi'ite cleric Moqtada al-Sadr urged Iraqis Sunday to protest the American presence and demand that Iraqi security forces stop cooperating with the U.S. military.

The threat of car bomb attacks prompted authorities to impose a 24-hour vehicle ban in Baghdad from five a.m. Monday (local time). Four years ago, U.S. forces swept into the Iraqi capital, ousting Saddam Hussein's government.

A White House spokesman, Gordon Johndroe, downplayed the anti-American protests, saying Iraqis are now free to gather and express their opinions, which he said they could not do under Saddam.

Iraq's Foreign Minister, Hoshyar Zebari told VOA Kurdish Service that April 9 is an historic day for Iraq, the region, and the world for ending what he called Saddam's "dictatorial regime."

Zebari said Iraq has made a lot of progress in the last four years, especially in the political field by advancing a democratic culture and establishing certain freedoms. He said many problems remain, but he said there has been some improvement since the launch of the new U.S.-Iraqi security operation.

Some information for this report was provided by AP and Reuters.