STORY: Outside the capital's Green Zone, which houses the parliament building that Sadr's supporters took over last week, the protesters threw stones at police. From behind concrete barriers, police responded by spraying them with water cannons.
The rival Shi'ite Muslim groups are affiliated with heavily armed militias, raising fears of clashes as tension soars over the failure to form a government nearly 10 months after an October election - the country's longest post-election deadlock.
Sadr's opponents include a grouping of parties and militias mostly aligned with Iran. That grouping, known as the Shi'ite Coordination Framework, said Monday's protest aimed to protect state institutions against the civil unrest of the Sadrists.
Sadr came first in the October election, but withdrew all his lawmakers from parliament after he failed to form a government that excluded his Shi'ite rivals.
He has since exerted political pressure through his masses of loyal followers, mostly working-class Shi'ites from poor neighborhoods in Baghdad and across southern Iraq, the heartland of the country's Shi'ite majority.
Sadr's actions have prevented his rivals, including bitter foe, ex-Prime Minister Nouri al-Maliki, from forming a government. Parliament must choose a president and premier and cannot convene while it is occupied by Sadr's followers.
The Sadrists have called for new elections and an end to the political system that has existed since the U.S.-led invasion which toppled Sunni dictator Saddam Hussein.