"The enemy that is occupying our country is now targeting the dignity of the Iraqi people," lawmaker Nassar al-Rubaie, head of Sadr's bloc in parliament, told the Associated Press during the march. "After four years of occupation, we have hundreds of thousands of people dead and wounded."
The protest, while not endorsed by the central government, is the latest test for Maliki, who came to power barely a year ago with the help of the 30 members of parliament affiliated with Sadr. It is a connection that has plagued the Bush administration's relationship with Maliki and caused many Sunni Muslim groups to question whether the Iraqi government is truly secular or a creature of the Shiite majority.
The spiritual and political leader for many Shiites opposed to the occupation, Sadr plays a critical role in the political process unfolding in Baghdad.
Sadr's supporters currently hold 30 seats in the elected 275-seat parliament and control six ministries in the Maliki government. Recent developments appeared to increase Sadr's importance to the fledgling government, rather than diminish it.
In early March, a Shiite Islamist party formerly allied to Maliki withdrew from Iraq's ruling coalition, saying it was ready to form a more cross-sectarian alliance.
The move by the Fadila or Virtue party does not put Maliki's government in jeopardy, but does make him more dependent on the other partners in his coalition, including the Sadrists. It also forces him to be closer with the second largest voting bloc, the Kurdistan Alliance.
Fadila spokesman Nadim al-Jabiri told a news conference that the party would "sit in parliament as an independent bloc awaiting moves from other political blocs to launch a patriotic agenda".
"We consider the first step of saving Iraq is to dismantle these blocs and to prevent blocs forming on a sectarian basis," he said in remarks reported by Reuters.
Fadila's criticism that Maliki has grown too sectarian and too influenced by groups like Sadr's has further stoked the fires of criticism and heightened concerns by U.S. officials, who have banked success of their Iraqi policy on the success of the Maliki government.
Sadr is a young cleric, but his family's name is closely associated with the fate of Iraqi Shiites going back generations. His father, the Grand Ayatollah Mohammed Sadiq al-Sadr, was the most powerful Shiite cleric in Iraq before he was assassinated with his other two sons in 1999 by agents presumed to be working for Saddam. After the dictator's fall in 2003, the urban, 2 million-person neighborhood in Baghdad was renamed from Saddam City to Sadr City.
In 2004, American forces clashed with Sadr's militia, the Mahdi Army, in a bloody series of street battles that left dozens of Americans and hundreds of Mahdi fighters dead. During the fighting, one of Sadr's senior aides, Ahmed al-Shibani, was arrested.
According to a U.S. military statement, al-Shibani was recently freed at al-Maliki's request and he "could play a potentially important role in helping to moderate extremism and foster reconciliation in Iraq."
The release has done nothing to tone down the rhetoric of Sadr and his supporters. Ahead of Monday's rally, Sadr's movement released a statement that urged his supporters to turn on the Americans and called on Iraqi forces to unite with his Mahdi militia to end the U.S. presence in Iraq. Despite his strong words, he stopped short of ordering a full-scale insurrection.
Sadr, who the U.S. military said fled to Iran when security operations began in Baghdad earlier this year, referenced the renewed fighting in the Diwaniya province -- clashes that have been the most violent in months between the Mahdi Army and the Americans. Many members of the Mahdi Army began moving to Diwaniya and other southern cities when the Baghdad crackdown began.
"My brothers at the Al Imam Al Mahdi army, my brothers in the security forces, enough fighting among you. This is giving success to our enemy's plans," the statement read.
Sadr's influence with the government has also troubled minority Sunnis who have accused the Maliki government of supporting, or at least allowing, Shiite militia violence that targets Sunnis.
"[O]ur relationship with the Al-Sadrist current and its leader Al-Sayyid Muqtada al-Sadr was good due to his patriotic position in the beginning that rejected the occupation, the political process, and the federation," Sheikh Harith al-Dari, head of the Sunni Association of Muslim Scholars in Iraq, told Lebanese newspaper Al-Safir in late March. "But he retreated, handed over the arms of his army to the government, and participated in the military operation. The militias of his army -the Al-Mahdi Army -became involved in the ethnic cleansing operations."
From the Maliki government there are also rumblings of frustration that no credit has been given for minimizing the clashes between the Mahdi Army and the Americans and for brokering several pieces of legislation like a new oil revenue sharing bill moving through the parliament.
"[The Americans] have said they are frustrated that he has done nothing to oust the Sadrists, that the oil law has not moved forward, that there is no genuine effort on reconciliation and no movement on new regional elections," an official told the Associated Press.
Some analysts in the region and in the United States have said the American frustration has led to a renewed flirtation with establishing a new governing coalition behind former Prime Minister Iyad Allawi.
The Financial Times reported in early March that the defection of the Fadila party "may be an attempt to reach out to other groups, such as the Sunni-led Iraqi National Dialogue Front (which won 11 seats in 2005) and the Iraqi National List of former prime minister Iyad Allawi (which won 25 seats)".
On Al Jazeera, Iraqi writer and political analyst Liqa Makki went even further, saying a government "overthrow" could be in the works and that talks Iyad Allawi held in Kurdistan in the process of searching for a new coalition were attended by U.S. Ambassador Khalilzad.
Whether the Allawi talks will turn into a political effort to build a less sectarian ruling coalition or not, the Maliki government, and its Sadr supporters, continues to rule. Monday's protests show the support for the fiery Shiite cleric has only intensified and soon may eclipse the political importance of the Baghdad government to many Shiite Iraqis.
"We get no help from Maliki. Only Sayyid Muqtada helps us," Saleh al-Ghathbawi, told the Washington Post in early April.