Named after an imam whose second coming has been awaited for a millennium, the Mahdi Army has become the most potent Shia force in Iraq.

Jaish al-Mahdi is the armed wing of Hojatoleslam Moqtada al-Sadr’s radical Islamist and nationalist grass-roots movement whose hallmark is vocal and violent opposition to the “occupation” by US and British troops in Iraq.

It has sought to portray itself to fearful Shias – successfully in many areas – as the one force strong and ruthless enough to defend their homes and neighbourhoods against al-Qaeda, Baathist and Sunni insurgents and their suicide bombings.

Although Hojatoleslam al-Sadr seeks to portray himself as a cross-community nationalist, his force has become an object of hatred and fear to many Sunnis, who accuse it of running death squads, with the goal of driving Sunnis from Baghdad.

It also provides welfare for the poor and other social services designed to win hearts and minds, as Hezbollah does in Lebanon and Hamas in the Palestinian territories.

Abu Sajaar, a senior Mahdi commander in Baghdad, told The Times that its image had been distorted by the Sunni and Western media. “We don’t attack anyone without a reason,” he said.

“Many times Sayyed Moqtada al-Sadr has said ‘My enemies are the takfiris[al-Qaeda extremists], the occupiers and the Baathists’. The occupiers are the worst, and after that the takfiris. The Baathists are finished now, but we hate them because the Baathists killed my brother and the occupiers killed my son.”

Hojatoleslam al-Sadr’s revered father, Mohammed Sadeq al-Sadr, a cousin and two of his brothers were murdered by Saddam during purges of the once-repressed and now dominant Shia majority. The family’s refusal to flee Iraq during the Saddam repression lends Hojatoleslam al-Sadr much of his street credibility in contrast to other Shias who went into exile.

While the political Sadrist wing has entered parliament – and until recently government – the Mahdi has infiltrated the police, army and Government, to the dismay of the US-led coalition seeking a transition to a moderate, secular and inclusive administration.

The Mahdi Army has had several confrontations with US troops. In recent months it has increased attacks against coalition forces, capitalising on widespread disillusionment at the Western failure to usher in security and prosperity.

The coalition has accused it of using Iranian-supplied roadside bombs to kill or maim Western troops and has arrested Mahdi and Sadrist leaders. Western officials say that recent attempts to isolate the extremist wing have succeeded in dividing the movement between factions loyal to Iran, Hojatoleslam al-Sadr and those pursuing criminal ends.

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