Widespread opposition to the deal has raised doubts that negotiators can meet a July target to finalize a pact to keep U.S. troops in Iraq after the current U.N. mandate expires at the end of the year.
The U.S. military, meanwhile, said an American Marine died Friday in a non-combat related incident in Iraq, pushing the number of Americans killed this month to 21 as May draws to a close.
While the number is not final, it would be the lowest monthly death toll since February 2004, when 20 troops died, according to an Associated Press tally based on military figures.
The Iraqi monthly toll also was down, with 516 violent deaths reported to the AP by police and other officials, the lowest since 375 were killed in December 2005.
Senior Sadrists, including lawmakers Falah Hassan Shanshal and Maha Adel al-Douri, met in the cleric's Sadr City office in Baghdad and issued a statement calling on the Iraqi government to stop negotiations with the U.S. and to hold a public referendum on the issue.
Al-Sadr, the hardline Shiite cleric and militia leader whose Mahdi Army battled American troops in Baghdad's Sadr City district until a truce this month, also has called for a referendum along with weekly protests against the deal. And, opposition has been growing among other groups.
U.S. and Iraqi officials began negotiations in March on a blueprint for the long-term security agreement and a second deal, to establish the legal basis for U.S. troops to remain in the country after a U.N. mandate runs out. Few details have been released about the talks.
Although U.S. officials insist they are not seeking permanent bases, suspicion runs deep among many Iraqis that the Americans want to keep at least some troops in the country for many years.
The U.S. military has continued to target what it calls Iranian-backed Shiite militia factions, warning key leaders have fled to other areas as American and Iraqi forces closed in on them in Sadr City.
American troops acting on tips in eastern Baghdad on Saturday captured a suspect believed to be a key assistant to one of the fugitive militia leaders, according to a military statement. The man captured was accused of kidnapping and managing funds for the so-called special groups.
Tensions also rose when Nassar al-Rubaie, the leader of the Sadrist bloc in parliament, was stopped at a police checkpoint outside Diwaniyah, south of Baghdad.
The six-car convoy, en route from Basra to the holy city of Najaf, was held up for nearly two hours without explanation, al-Rubaie told AP in a telephone interview. He called for the government to stop harassing Sadrists and put those responsible on trial.
Police Col. Asaad Ali, the director of the Diwaniyah operations center, said police stopped the convoy because gunmen are not allowed in the city and al-Rubaie was protected by armed guards. He said a patrol was sent to safely escort the convoy on its way out of the province.
Separately, French Foreign Minister Bernard Kouchner expressed renewed commitment to helping the Iraqis rebuild as he made his second visit to the wartorn country in less than a year.