BAGHDAD - Tens of thousands of militiamen and supporters of anti-American cleric Muqtada al-Sadr marched in Baghdad Thursday in a dramatic show of strength, saying to both the U.S. and Iraqi governments: If American troops stay past Dec. 31, there will be violence.

"I am asking for the withdrawal of the occupation. I am ready to fight from this moment. I am ready to sacrifice. I am ready for death," said one of the marchers, 42-year-old Hussein Abu Lika.

Under an agreement between Washington and Baghdad, the 46,000 troops still in Iraq must leave by Dec. 31. But Iraq's widespread instability and still struggling security forces have led U.S. and Iraqi leaders to reconsider the deadline for the sake of the country's security.

U.S. officials have been pushing Iraq to decide whether it wants some American forces to remain, and Prime Minister Nouri al-Maliki has said he'll discuss it with the country's main political blocs.

But the throngs on the street in Sadr City, a slum in eastern Baghdad that is an al-Sadr stronghold, was a stark warning to al-Maliki about what he risks if U.S. forces stay longer.

The 37-year-old al-Sadr grudgingly threw his support behind al-Maliki last year, helping him clinch a second term in office, but he is vehemently against any continued U.S. military presence. Already the cleric has threatened to unleash his Mahdi Army if U.S. forces stay in 2012.

Al-Sadr did not appear during the 3 1/2 hour-long rally that ended just at prayer time. Adoring crowds surged at a convoy believed to be carrying al-Sadr, but it drove away and it was unclear if al-Sadr was in one of the vehicles. At least one al-Sadr aide who did not want to be identified because he was not authorized to speak to the media said he was but would not say why he did not get out of the vehicle.

Despite al-Sadr's absence, almost every Sadrist lawmaker and political and religious leader of note was in attendance at the demonstration, watching as rows and rows of militiamen marched down one of Sadr City's main thoroughfares.

Al-Sadr is one of the few, maybe only, Iraqi political leaders able to rally such a large crowd. Many of them are impoverished Shiites from southern Iraq and Baghdad who are drawn to his anti-American, nationalist rhetoric and his family's deep roots in Iraqi political and religious life.

But to many Iraqis al-Sadr and his Mahdi Army are little more than thugs blamed for some of the worst of the sectarian attacks during the insurgency.

The marchers, wearing matching T-shirts and caps with the Iraqi flag emblazoned on them, also waved Iraqi flags and shouted "No, no, America!"

U.S., Israeli and British flags were painted on the pavement to be stomped on by the marching protesters, and Iraqi military helicopters buzzed overhead while soldiers stood guard. Organizers said at least 700,000 militiamen and supporters were in attendance while the U.S. military put the number closer to 20,000.

Though the rally was billed as a peaceful demonstration, a close aide to al-Sadr, Salah al-Obeidi, said threats against the U.S. still stand if the troops stay.

"We will be obliged to fight and do our best to liberate our country," al-Obeidi said.

Already American forces in Baghdad and southern Iraq have seen an increase in rocket and mortar attacks and roadside bombs. U.S. officials have blamed the uptick on Shiite militias backed by Iran who are trying to take credit for driving American forces from Iraq.

In comments to the BBC, al-Sadr said his followers continue to launch operations against American troops and target their bases.

Iraqi security forces also come under attack although usually by Sunni extremists. On Thursday, a suicide bomber struck an Iraqi army patrol in western Baghdad, killing four soldiers and wounding 11 others, officials said. They spoke on condition of anonymity because they were not authorized to speak to the media.

Al-Sadr inherited the mantle of the popular Shiite movement from his father, who was beloved by followers and killed by Saddam Hussein's regime. The younger al-Sadr left Iraq in 2007 and took up residence in neighboring Iran, partly to study religion and to avoid an arrest warrant for him at the time.

His return to Iraq in January capped a dramatic return to prominence on the Iraqi political scene. His political party now controls 40 seats in the Iraqi parliament.

Ammar Abdul-Muheimin, a 44-year old Sunni resident of Baghdad's western Amiriyah neighborhood said the Sadrists' growing power might destabilize Iraq.

"Now they are in the political process, and they will not hesitate to use their influence in the street, whether by violence or peaceful means, to protect their political gains," he said.