A day after Defense Secretary Robert Gates suggested that American troops could remain here for years, tens of thousands of protesters allied with Muqtada al-Sadr, the radical anti-American Shiite cleric, flooded the streets demanding an end to the American military presence.

The protests were scheduled before Gates' comments - made Friday during a visit to troops in northern Iraq - although his statements may have fueled some of the day's fervor. The protesters were whipped up by comments drafted by al-Sadr, who sent a message to the crowd threatening to reconstitute his militia, the Mahdi Army, if the American military did not leave this year.

A demonstration against the American invasion is held each April 9, the anniversary of the fall of Baghdad in 2003. But the event took on more political importance this year because it came amid the debate about whether American troops will leave on schedule by the end of the year or stay on in some capacity. The departure date was set by a security agreement that binds both countries.

Under the terms of the security agreement, the Iraqi government would have to ask the United States to stay on. But any such request would be politically complicated for Prime Minister Nouri al-Maliki because of al-Sadr's influence.

President Obama has said that all troops will leave by the end of the year, but Gates' comments Friday amounted to a rare public acknowledgement of what many officials say privately: that there is much behind-the-scenes deliberation about a continued presence of U.S. forces after this year. Gates mentioned the possibility of keeping troops in what he called an "advise-and-assist" role.