Militiamen and followers of anti-American cleric Muqtada al-Sadr led a massive rally on Thursday, marching in Baghdad in a show of force as Iraqi leaders weigh whether to keep U.S. troops in the country beyond the end of the year.

At least tens of thousands waved Iraqi flags and shouted "No, no, America!" as the tight columns of the unarmed but ominous-looking members of the Mahdi Army, as al-Sadr's militia is known, marched through one of Baghdad's poorest neighborhoods.

"I am ready to fight the Americans whenever Sayyid (Muqtada) orders me to," Mohammed Moyad, 18, who said he skipped five days of school to train with his colleagues for Thursday's march.

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 to keep peace if needed. Organizers said at least 700,000 militiamen and al-Sadr supporters had taken to the streets while the U.S. military estimated the crowd at about 70,000.

The rally was a message to Prime Minister Nouri al-Maliki about the staunch opposition by Iraq's most devout Shiites _ and the ones who grudgingly helped him clinch a second term in office last year _ to a continued U.S. military presence in 2012.

Under a security agreement between Washington and Baghdad, the 46,000 combat troops still in Iraq are required to leave by Dec. 31. But Iraq's widespread instability has led U.S. and Iraqi leaders to reconsider the deadline for the sake of the country's security.

Al-Sadr had not yet appeared nearly two hours after the start of the march, and his top aide, Salah al-Obeidi, said the cleric likely would not. Adoring crowds surged at a convoy of more than 10 white sport utility vehicles that was believed to be carrying al-Sadr, but it drove away without stopping.

Though the rally was billed as a peaceful demonstration, 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," he said.

Already, American forces in Baghdad and southern Iraq have seen an increase in rocket and mortar attacks as well as roadside bombs in recent months. 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.

Al-Obeidi said the point of the rally was to show that Iraqis are disciplined and can protect the country. A statement by parliament Speaker Osama al-Nujaifi, a Sunni, called the march "clear proof to Iraq's unity."

However, al-Nujaifi spokesman Mohammed al-Khalidi sought to distance the speaker from the specter of violence. Al-Khalidi said his presence at the rally did not necessarily mean al-Nujaifi opposes the U.S. presence troop but declined to elaborate.

U.S. officials counted more than 300 busloads _ each carrying up to 70 passengers - who traveled from Iraq's south for the rally, and were joined by some of the roughly 2 million who live in Baghdad's northeast Sadr City neighborhood where it was held.

An estimated 24,000 militiamen from southern provinces wore matching T-shirts bearing the Iraqi flag as spectators burned American and Israeli banners. Small groups of youths along the parade route also struck Americans flags with twirling kickboxing moves to the delight of onlookers.

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Associated Press writers Rebecca Santana and Lara Jakes contributed to this report.