Thousands of followers of Shiite cleric Muqtada al-Sadr converged on a central Baghdad square Friday for a mass prayer to protest a proposed U.S.-Iraqi security pact.

The demonstrators waved Iraqi flags and green Shiite banners, chanting, "No, no to the American agreement!" and, "No, no to the agreement of humiliation!"

Iraq's parliament is expected to vote Monday on the plan to keep U.S. forces in Iraq for another three years. But the noisy opposition by the Sadrists indicates that even if it is approved, the deal could remain divisive in a country struggling for reconciliation.

Opponents view the security deal as a surrender to U.S. interests despite Prime Minister Nouri al-Maliki, a Shiite, saying the pact would eventually lead to full sovereignty.

Al-Sadr, who is believed to be in Iran, was not at the protest, though he wrote a sermon read by his representative, Sheik Abdul Hadi al-Mohammadawi, calling the U.S. "the enemy of Islam."

"The government must know that it is the people who help it in the good and the bad times. If it throws the occupier out all the Iraqi people will stand by it," the sermon read, using common rhetoric for the United States.

Al-Sadr reiterated in the sermon that his followers in both the armed and the peaceful factions of his movement will continue to work for the removal of U.S. forces.

The protest was being held at Firdous Square, where U.S. Marines tore down a statue of Saddam Hussein in one of the iconic images of the U.S.-led invasion in 2003.

Organizers placed an effigy of President George W. Bush holding a sign that said, "The security agreement ... shame and humiliation" on the pedestal near a sculpture that replaced the Saddam statue.

Many protesters arrived at the square on foot or by bus and carried prayer rugs, pieces of cardboard or newspapers for the mass prayer.

Security was tight, with the area closed to unofficial traffic and heavily guarded by Iraqi security forces.

The Cabinet has approved the agreement, meaning it stands a good chance of passage in the 275-seat parliament where the government's parties dominate. But for al-Maliki's Dawa party and the Supreme Islamic Iraqi Council, its senior government partner, the margin of support is almost as important as the victory itself. A narrow vote for approval will cast doubt on the legitimacy of the new terms governing the U.S. troop presence.

Al-Sadr's followers and other legislators opposed to the pact also try to could use the narrow vote to turn their anti-American message into a defining issue in provincial elections on Jan. 31 and general elections late in 2009.

If the agreement passes the legislature, it will go to the president and his two deputies for ratification. Each one has veto power.