The scion of one of Iraq's most powerful Shiite clerical families, Muqtada al-Sadr has emerged as one of the most potent political and military leaders in post-Saddam Hussein Iraq and as one of the most vocal critics of the U.S. occupation.

Al-Sadr is in many ways a paradox, preaching a largely nonviolent message while heading the al-Mahdi Army, an armed militia that has repeatedly clashed with U.S. and Iraqi government forces.

Believed to be in his early thirties, al-Sadr's hails from the holy Shiite city of Najaf. Although he lacks the decades-long religious training required of the highest-ranking Shiite authorities, based on his lineage, leadership of the rebellion and popular support al-Sadr has established himself as one of the top religious leaders in the war-torn nation. His followers have begun to say he is a hujjat al-Islam -- a "Sign of Islam," or a "Proof of Islam," the third rank from the top in the Shiite clerical hierarchy and he often wraps himself in a white funeral shroud, showing he is ready for death.

But for an al-Sadr to achieve such a following and play such an influential role is nothing new. His father, the Grand Ayatollah Mohammed Sadiq al-Sadr, was the most powerful Shiite cleric in Iraq before he was assassinated along with his other two sons in 1999 by agents presumed to be working for Saddam.

Already a popular figure at the time of his death, the murder of Grand Ayatollah al-Sadr transformed him into one of the major symbols of Shia resistance to the former regime.

But the al-Sadr family had been a target of Saddam's government for years. Muqtada al-Sadr's uncle, Ayatollah Mohammed Baqir al-Sadr, was another Shiite leader executed by Saddam's forces in 1980.

Following an attempt on his life in February 1999, Muqtada al-Sadr went underground. In the wake of his father's death, al-Sadr inherited a network of schools and charities built by his father, along with the allegiance of many of the elder Sadr's followers.

Experts estimate Muqtada al-Sadr commands the loyalty of some 3 million to 5 million Shiites across the country; a following that has swelled with each confrontation with the United States. Many of his supporters live in Sadr City, a vast Baghdad slum of 2 million previously called Saddam City and renamed for the senior Sadr after Saddam's fall. Sadr also has strong support in Najaf, the holy city where the cousin and son-in-law of the Prophet Mohammed, Ali ibn Abi Talib, is buried.

Al-Sadr supporters claim his al-Mahdi Army numbers 10,000 men, although some reports place the number between 1,500 and 3,000. Some experts say there also are hundreds of thousands of additional Iraqis who are "passive" al-Sadr supporters, who admire his outspoken opposition to the U.S. role in Iraq, respect his heritage and benefit from the services his social network provides but are not willing to stand up to the Americans on his behalf.

From the outset of the American occupation, al-Sadr has called for their withdrawal. He rejected the U.S.-appointed Iraqi Governing Council and has advocated a so-called "faithfully Islamic government".

On March 28, 2004, the U.S. occupation authorities ordered the closure of al-Hawza newspaper, published by al-Sadr, alleging it was inciting violence. Declaring that peaceful protests had become useless, al-Sadr urged his followers days later to "terrorize" their enemy.

Seen as the worst outbreak of Shia resistance in the year-old U.S.-led occupation of Iraq, al-Sadr's backers demanded the reopening of their newspaper. In the fighting hundred of Shiite fighters and dozens of Americans died in more than a week of combat.

Iraqi interim Prime Minister Iyad Allawi issued a decree the following July to restart the newspaper.

The cleric started a campaign for unity among Muslims following an explosion of the Shiite Golden Mosque in Samarra in late February 2006, which sparked a wave of reprisal attacks against Sunnis.

Al-Sadr accused Iraq's enemies of fomenting sectarian strife.

"I call upon all believers, Sunnis and Shiites, to unite. All Iraqis should be brothers to each other," he said.

At the same time, reports emerged that his militias were conducting revenge attacks on Sunnis opposed to a Shiite dominated government.

Although he lacks the religious education required under Shia law to be a cleric, he has set himself up as a major force in the country's Shiite community by radical rhetoric and control of the al-Mahdi militia.

-- Compiled by Kathryn Cohen for the PBS NewsHour