At times he has called for a national rebellion against foreign troops and sent out his militiamen to confront the "invaders" and Iraqi police.
At others he has appeared more compromising, seeking for himself a political role within the new Iraq, and his political followers have 30 MPs and a handful of ministers in the American-backed government.
His militia the Mehdi Army has also observed a ceasefire since August 2007 that is widely credited with reducing sectarian tensions and contributing significantly to the fall in violence.
He is thought to be in his early to mid-30s - a youthful leader in a society which considers age and experience essential to religious authority.
Moqtada Sadr mixes Iraqi nationalism and Shia radicalism, making him a figurehead for many of Iraq's poor Shia Muslims.
His detractors see him as an inexperienced and impatient radical who aims to dominate Iraq's most revered Shia institutions by force.
The youngest son of Muhammad Sadiq Sadr - a senior Shia cleric assassinated in 1999, reportedly by agents of the Iraqi government - Moqtada Sadr was virtually unknown outside Iraq before the US-led invasion in March 2003.
The collapse of Baathist rule revealed his power base: a network of Shia charitable institutions founded by his father.
In the first weeks following the US-led invasion, Moqtada Sadr's followers patrolled the streets in the poor Shia suburbs of Baghdad, distributing food.
The Sadr name clearly has powerful resonances; the Shia district of Baghdad, Saddam City, was renamed Sadr City after the fall of the Iraqi leader.
Armed force
In June 2003 he established a militia group, the Mehdi Army, in defiance of coalition arms controls, pledging to protect the Shia religious authorities in the holy city of Najaf.
"Terrorise your enemies as we cannot remain silent at their violations"
He also set up a weekly newspaper, al-Hawzah. The US-led authorities imposed a ban on the paper in March 2004, accusing it of inciting anti-US violence.
The ban was later lifted by the interim Iraqi government, citing the move as proof of its belief in a free press, but the newspaper said it had been preparing to resume publishing anyway.
In contrast to more moderate clerics such as Grand Ayatollah Ali Sistani, Moqtada Sadr calls on Shia spiritual leaders to play an active role in shaping Iraq's political future.
He has also been noted in the past for his fiery rhetoric against the presence of foreign troops.
The radical cleric has led uprisings against US forces centred on Najaf which saw weeks of fighting in 2004, which only abated after the mediation of Ayatollah Sistani.
As insurgent attacks have increasingly targeted Shia areas, the Mehdi Army has become one of the major armed forces on the ground in Baghdad, controlling - and protecting - predominantly Shia areas.
Sunnis have accused its members of carrying out sectarian killings, although Mr Sadr denies the accusations.
Political move
An Iraqi judge has released an arrest warrant for Moqtada Sadr in connection with the death of a moderate Shia leader, Abdul Majid al-Khoei, in April 2003, just two days after the fall of Baghdad.
Moqtada Sadr strongly denies any role in the murder.
His supporters have also clashed with followers of Ayatollah Sistani.
He has visited neighbouring Iran since Saddam Hussein was ousted, meeting senior officials in Tehran.
Moqtada Sadr's appeal to the Shia poor and dispossessed accounts for much of his popularity. However, some other Iraqis also see him as symbol of resistance to foreign occupation.
Followers of the cleric stood for parliament as part of the United Iraqi Alliance bloc of Shia political groups in December 2005.
But in November 2006, he declared a boycott of the Iraqi government in protest at Iraqi Prime Minister Nouri Maliki's summit with US President George W Bush in Jordan.
The boycott lasted for two months, with the Sadrists eventually returning to their positions.
In April 2007, Mr Sadr called a massive rally in Najaf, demanding Mr Maliki agree a timetable for withdrawal of US-led troops in Iraq.
The firebrand cleric did not appear in person at the rally adding fuel to US claims - denied by his supporters - that he had left the country and was in hiding in Iran.
When the government did not respond to the call, he distanced himself from it by pulling out his six cabinet ministers for an indefinite period, although his parliamentary bloc did not leave the governing coalition.
BBC Monitoring, based in Caversham in southern England, selects and translates information from radio, television, press, news agencies and the Internet from 150 countries in more than 70 languages.
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