Chanting "no to America," supporters of a radical Shiite cleric burned an effigy of President George W. Bush Friday in a protest demanding parliament scuttle a U.S.-Iraqi security pact and American troops begin withdrawing from Iraq immediately.

The demonstration drew nearly 20,000 followers of Muqtada al-Sadr to Firdous Square, the same spot where U.S. Marines toppled a statue of Saddam Hussein and exultant Iraqis pummeled it with debris in what became an iconic image of the fall of Baghdad and the end of the dictator's 23-year rule.

Friday's protest was the latest display of opposition to an accord that could push Iraq into new political turmoil even though the ruling coalition appears to have enough parliamentary votes to narrowly approve the deal.

Parliament is scheduled to vote on the pact Monday, but presidential spokesman Naseer al-Ani told Iraq's Sharqiyah television that the vote might be delayed until after the Muslim feast of Eid al-Adha, which falls in early December.

"It will need more time. Perhaps until after Eid al-Adha," he told the station. The legislature is expected to go into recess this month ahead of Eid al-Adha, when scores of lawmakers travel to Saudi Arabia for the Muslim pilgrimage to Mecca.

The pact establishes a timeline for the withdrawal of U.S. forces from cities by June 30 and the entire country by 2011. It places U.S. forces under tight Iraqi control and gives the Iraqis limited powers to put American soldiers and civilian Pentagon employees on trial in cases of serious crimes committed off-base and off-duty.

Prime Minister Nouri al-Maliki has said the agreement offers Iraq the only viable option to regain full sovereignty. The alternative would be to seek the renewal of a U.N. mandate that, he said, allows U.S. forces a free rein in the country. The mandate expires Dec. 31.

But none of that mattered Friday at Firdous Square, where protesters waved Iraqi flags and green Shiite banners and chanted: "No, no to the agreement of humiliation!"

Al-Sadr, who is based in Iran, did not attend. But in a sermon read to the crowd by an aide, he criticized the government and described America as "the enemy of Islam."

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

Organizers placed an effigy of Bush on the same pedestal where the giant Saddam statue stood before it was knocked down on April 9, 2003. A sign attached to the effigy described the pact as "shame and humiliation."

After a mass prayer, demonstrators pelted the Bush effigy with plastic water bottles and shoes. One man standing on the pedestal hit it in the face with his sandal.

The effigy fell into the crowd and protesters jumped on it before setting it ablaze as the crowd erupted with chants of "Allahu Akbar," or "God is great." Several U.S. flags were also burned.

The demonstration followed two days of raucous protests in the 275-seat parliament by al-Sadr loyalists who disrupted readings of the proposed pact by shouting and pounding their desks.

Al-Sadr's influence in Iraq has dipped compared to the days when his militiamen battled U.S. forces in Baghdad and across southern Iraq in 2004, and when they were seen as protectors of Shiites against Sunni militants at the height of the sectarian violence in 2006 and 2007.

His movement's popularity suffered with the involvement of some militiamen in protection and black market rackets, as well as general fatigue from the on-again, off-again fighting. It has retained a loyal base of support in Baghdad and the Shiite south, largely because of nationalist credentials and the perceived failure of rival Shiite parties to improve services.

Al-Ani, the presidential spokesman, acknowledged al-Sadr's group enjoys the sympathy of "not an insignificant segment" of the population.

"As long as they remain a part of the political process, they can say what they want," he said after President Jalal Talabani and representatives of several political blocs met to discuss the security pact.

If the vote were held Monday, the pact seems headed to a narrow victory in the fractious legislature, a prospect that could deepen Iraq's political divisions and deal a serious setback to reconciliation efforts.

The country's most influential Shiite cleric, Grand Ayatollah Ali al-Sistani, has indicated the agreement would only be acceptable if it passes with a big majority.

But that seems unlikely now. With all votes from parliament's main Shiite and Kurdish blocs — the senior partners in al-Maliki's coalition — the government can muster just over 140 seats, a few above the simple majority threshold.

Such a narrow margin would cast doubt on the pact's legitimacy and could prompt al-Sistani to speak against it. If he does, the agreement would be buried.

It is unclear how the government's Sunni Arab partners, the Iraqi Accordance Front, will vote. Its 44 lawmakers could give the government the respectable margin of victory it seeks, but leaders of the bloc are making their approval conditional on a package of reforms to give their once-dominant community a bigger say in running the country.

Al-Maliki said he was surprised by the Sunni demands and suggested they were tantamount to political blackmail.

The Sadrists, who have 30 lawmakers in parliament, are leading the camp that opposes the security deal, which includes the Shiite Fadhila party with 15 seats and a small Sunni bloc with 11 seats. There is no firm word on how the remaining 30 or so lawmakers would vote.

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

Senior members of al-Sadr's movement acknowledge the agreement is likely to pass, but see gains in the political storm around it.

"If the agreement passes with a small majority, it will be a defeat for those who sponsored it," said Salah al-Obeidi, al-Sadr's spokesman.

The Sadrists are looking beyond the agreement, hoping their intense opposition will translate into votes in provincial elections slated for Jan. 31 and a general election late next year.