Protesters shout slogans as a burning American flag is waved Friday during a rally in the Shiite enclave of Sadr City, Iraq.

Baghdad ? Two Shiite militia leaders surrendered to American soldiers Friday, while tens of thousands of supporters of hard-line Shiite cleric Muqtada al-Sadr streamed out of mosques to protest against an agreement which could keep U.S. troops here for years.

The arrests and demonstrations occurred on the eve of Prime Minister Nouri al-Maliki’s trip to Shiite-dominated Iran, his second visit there in a year.

U.S. officials allege that Iran is arming and training Shiite militiamen and encouraging a public campaign in Iraq against the proposed U.S.-Iraq security agreement, which the Iranians oppose.

One of those who surrendered early Friday allegedly ordered attacks on U.S. troops, directed the kidnapping of Iraqis and helped smuggle Iranian weapons into Iraq, the U.S. military said in a statement.

The other tried to escape by wading through an irrigation canal before turning himself over to U.S. soldiers.

Names of the suspects were not released, but both were members of Iranian-backed “special groups,” the U.S. command said. The term is used by the American military to describe Shiite fighters who have defied al-Sadr’s cease-fire order that ended seven weeks of fighting in Baghdad last month.

Iran denies arming the extremists, and it is unclear whether significant numbers of al-Sadr’s Mahdi Army militia are really beyond his control.

U.S. and Iraqi troops have been trying to block the movement of Iranian weapons into Iraq through a series of raids in mostly Shiite areas south of Baghdad.

Allegations of Iranian links to Shiite militants and the proposed U.S.-Iraqi security agreement are expected to figure prominently in al-Maliki’s talks, which begin today. No timetable for the visit has been released because of security concerns.

The deal, which the Iraqis and Americans hope to finish by midsummer, would establish a long-term security relationship between Iraq and the United States. The two sides are also negotiating a parallel agreement to provide a legal basis for keeping U.S. troops in Iraq after the U.N. mandate expires at the end of the year.

Supporters believe the deal would guarantee U.S. support as Iraq seeks to cement the security gains of the past year. It would also help assure Iraq’s Arab neighbors, notably Saudi Arabia, that Iraq’s Shiite-led government would not become an Iranian satellite.

U.S. officials have released no details about the negotiations, which began last March.

But leaks by Iraqi authorities have triggered a storm of protest, with critics complaining that the terms would solidify American military, economic and political domination of the country for decades.

Iraqi officials also complain that the deal would enable U.S. troops and officials to continue to enjoy immunity from prosecution under Iraqi law and allow the Americans to conduct military operations without clearing them with the Iraqi government.

Al-Maliki’s Dawa party has described the talks as stalled, and prominent parliamentarians from Shiite, Sunni and Kurdish parties have written to the U.S. Congress to express their opposition to the proposed agreement, which must be approved by the Iraqi legislature.

But the most outspoken opponents have been from al-Sadr’s political movement, which has long opposed the presence of foreign forces on Iraqi soil.