He is seen by many Shia and politicians as a zealous leader who has chosen the wrong time for this escalation of protests.
About 30 years old, al-Sadr is a son of the Grand Ayat Allah Muhammad Sadiq al-Sadr, a prominent Iraqi Shia leader who was killed in 1999 along with two of his other sons.
Hardly known outside Iraq, and lacking the religious education and degrees required by Shia doctrines, al-Sadr bases his religious authority on his lineage.
Saddam Hussein backed his father, Muhammad Sadiq al-Sadr, to head the al-Hawza (the main centre for Shia instruction in Iraq) in Najaf. Hussein backed al-Sadr because he was an Arab Muslim and he wanted to rid the al-Hawza of its non-Arab leaders.
Muhammad Sadiq al-Sadr used that backing to consolidate his hopes to change Iraq's religious, political, and social outlook to comply with Islamic rules.
"I do not believe that
any sensible human being would kill a leader such as Muhammad Sadiq al-Sadr"
former Iraqi foreign minister
He reportedly gained popular support to an extent that worried the Iraqi government. Unconfirmed reports suggest he has been killed by the Iraqi secret services.
But sources close to former Iraqi foreign minister Naji Sabri quoted Sabri as saying before the war on Iraq: "I do not believe that any sensible human being would kill a leader such as Muhammad Sadiq al-Sadr."
Continuing his father's attempt to lead the al-Hawza in Najaf, his followers surrounded the home of Grand Ayat Allah Ali al-Sistani, an Iranian citizen, asking him to leave the country soon after occupation. This followed the murder of the prominent Shia cleric Abd al-Majid al-Khoei who had returned to Iraq with US forces after years in exile.
Muqtada al-Sadr found a great deal of space for manoeuvre - as is the case with all Iraq’s political and religious factions. He consolidated his power base among his father's supporters and started a conscious anti-occupation campaign.
He has repeatedly expressed opposition to the US-led occupation of Iraq; however, he has preached non-violence.
He did not recognise the US-appointed Iraqi Governing Council and declared a shadow cabinet.
Al-Sadr founded paramilitary militias, al-Mahdi Army, saying it would fight for the interests of the Iraqi people.
On 28 March 2004, the US occupation authorities ordered the closure of al-Hawza newspaper, published by Muqtada al-Sadr, alleging it was inciting violence.
Al-Sadr's followers demonstrated in their thousands in several cities, protesting against the closure.
Peaceful protests across Iraq
have escalated tensions
Declaring that peaceful protests had become useless, al-Sadr urged his followers to "terrorise" their enemy on 5 April 2004.
His call came only one day after thousands of his followers took to the streets in protest. Armed supporters, mostly impoverished young men belonging to al-Mahdi Army, have reportedly engaged in gun battles with coalition forces.
Eight American soldiers, one Salvadorean combatant and at least 20 Iraqi demonstrators were reported killed in the day-long fighting.
Seen as the worst outbreak of Shia resistance in the year-old US-led occupation of Iraq, al-Sadr's backers demanded the reopening of their Al-Hawza newspaper.
Al-Sadr's supporters also demanded the end of a siege that was imposed on al-Sadr's offices and the release of Mustafa Yaqubi, a top aide who was arrested a week earlier.
Muqtada al-Sadr has maintained his rejection of the US-appointed Iraqi Governing Council and has actively advocated the so-called "faithfully Islamic government".
A spokesman for the US-led occupation of Iraq has said an arrest warrant has been issued for al-Sadr.