The radical group Jamaat al-Sadr al-Thani has been building a power base in the Shia holy city of Najaf in opposition to the conservative Shia leadership and returning exiles there.
The group, known as the "Sadr group", is led by Muqtada Sadr, the son of a Shia cleric killed by the Baathist regime.
Mr Sadr opposes co-operation with the US-led occupation. His sermons mix calls for the application of Islamic law with appeals to Iraqi national pride.
His supporters have been accused of playing a role in the murder of a moderate, pro-Western Shia cleric in Najaf in an outbreak of fighting shortly after Saddam Hussein's regime fell.
They were also widely accused of besieging the home of Grand Ayatollah Ali al-Sistani, the most senior Shia cleric in Iraq, demanding that he leave the country. Mr Sadr has denied both claims.
Mr Sadr has said that 10,000 young men have come forward to join an "Islamic army" in Najaf.
His name clearly has powerful resonances - the Shia district of Baghdad, Saddam City, has been renamed Sadr City.
Mr Sadr has criticised Iranian influence in Iraq, despite having gained an audience with senior Iranian figures, including the Supreme Leader Ayatollah Khamenei, on recent visits.