BAGHDAD, Iraq (CNN) -- An agreement between the United States and Iraq to allow U.S. troops to remain operating in Iraq past 2008 should be put to a popular referendum, Shiite cleric Muqtada al-Sadr urged in an online message to his followers.
A Baghdad street is reflected in the glass over a poster of Iraqi Shiite cleric Muqtada al-Sadr.
The message also calls for weekly protests against the agreement, being negotiated by the two governments.
Al-Sadr also called for "an organized media action" and "a unified political and parliamentary movement" to oppose the standards of forces agreement, which would replace the U.N. resolution that allows U.S. troops to operate in Iraq when it expires at the end of the year.
Al-Sadr's Mehdi Army militia has frequently clashed with Iraqi and U.S. forces, most recently in Sadr City, a Baghdad neighborhood considered a Sadrist stronghold.
In the statement posted on a loyalist Web site, the popular cleric renewed his call for a timetable for the departure of U.S. troops and called for delegations to approach the United Nations, the European Union, the Arab League and other Iraq neighbors to discuss the agreement.
Protests against the agreement should be carried out after Friday prayers "until further notice or until the treaty is canceled," al-Sadr said.
If the government rejects his call for a referendum on the agreement, al-Sadr will order his offices "to work on collecting millions of signatures opposing" it, the message said.
The United States and Iraq began negotiating the agreement this year but have not publicly discussed its contents except in vague terms.
Last month, U.S. Ambassador to Iraq Ryan Crocker told the Senate Foreign Relations Committee that the agreement will cover the "basic authorizations and protections" to allow troops to continue operations but will not specify troop levels or establish permanent bases.
"We anticipate that it will expressly forswear them ... and it will not tie the hands of the next administration," he said.
In January, as the negotiations were about to begin, Defense Secretary Robert Gates called the agreement "an approach to normalizing the relationship between the United States and Iraq."
Senior Defense officials said in January that the agreement will spell out terms for the U.S. presence in Iraq after December 31, when the U.N. mandate expires.
"There will be a need to make sure our forces in Iraq have the tools they need to be able to do the job they need to," one official said. "Our forces need to be able to defend themselves."
Such tools also include the authority to detain terrorist suspects and keep the ones already detained in captivity, the officials said.
Some members of Congress have objected to the Bush administration's plans to handle the agreement without seeking congressional approval.
CNN's Yousif Bassil contributed to this report.