|Mahdi Army fighters have repeatedly clashed with US and Iraqi forces [GALLO/GETTY]|
The Mahdi Army is an armed group loyal to Muqtada al-Sadr, a Shia leader from a family line of revered clerics persecuted under Saddam Hussein - Iraq's former president.
The group was formed in 2003 to protect Shia areas due to the collapse of public order in the aftermath of the US-led invasion of Iraq.
Its members are often popular in the neighbourhoods they control because the group offers services that the Iraqi government is often unable to provide.
"This is an army of volunteers ... They are clerics at night and heroes during the day," said Abu Bakr, a resident of Baghdad's Sadr City district.
"This army is helping society. They clean the streets, protect our schools and distribute fuel and gas."
Sadr City is one of the group's strongholds and there the Mahdi Army has banned black markets, which are rampant in the rest of the capital, and members man strict neighbourhood security checkpoints to search for car bombs.
"Ask anyone around," one of its fighters said, "they will tell you that without our presence, they will not be able to sleep at night, [and] students will not be able to go to school, like in the rest of the capital, where people are scared."
The group rose to international prominence on April 4, 2004, when it spearheaded the first major armed confrontation against the US forces in Iraq, in an uprising that followed the banning of al-Sadr's newspaper and attempts to arrest him. The uprising lasted until June.
The Mahdi Army began as a small group of roughly 500 religious students connected with Muqtada al-Sadr in Sadr City.
The Iraq Study Group, also known as the Baker-Hamilton Commission, last year estimated that the force had 60,000 members, but others put the number much larger, saying that the Mahdi Army is present in every city and town - from Baghdad to the southern border with Kuwait.
Al-Sadr is against the presence of foreign troops in Iraq and has demanded a timetable for the withdrawal of US forces.
|US forces first clashed with the Mahdi Army in|
2004 when it rose up in revolt [GALLO/GETTY]
He told Al Jazeera that the Mahdi Army will only disarm when an administration that can "get the occupier out of Iraq" is present.
The Mahdi Army is capable of "liberating Iraq", he said, maintaining that the US-backed government is as "distant" from the Iraqi people as Saddam Hussein's.
Many Sunnis are fearful of the group, which they accuse of carrying out a relentless campaign against them.
Abdullah, a Sunni student in Baghdad, told Al Jazeera: "If anyone from them [the Mahdi Army] recognised that I am Sunni, then I will be targeted."
The group is accused of infiltrating the security forces and its members have reportedly used police uniforms to set up fake checkpoints and hunt down Sunnis.
The Mahdi Army had in the past concentrated on fighting US troops, and on two occasions sent aid to Sunni fighters in Falluja during military offensives led by US forces.
But that support dried up in February 2006, when the Askari mosque, a holy site for Shia Muslims in Samarra, was bombed. Within hours of the bombing, young people were riding around the capital on the back of pickup lorries, parading guns and vowing revenge.
Al-Sadr, however, insists that Sunni fighters are allies of the Mahdi Army and that he stands with them politically.
"I am an admirer of the Sunnis and one of them," he told Al Jazeera.
Accused of being influenced by Shia neighbour Iran, al-Sadr says he has told the country's supreme leader, Ayatollah Ali Khameini, that he does not approve of the "political and military interests" that Tehran's government has pursued in Iraq.
Mahdi Army commanders, though, say they have accepted arms and cash from Iran.
Last year saw a significant drop in violence across Iraq, largely due to a ceasefire between the government and the al-Sadr's followers, according to the US military.
However, a recent bout of fighting has resulted in fighters loyal to al-Sadr locked in battles with US and Iraqi forces. An operation against Shia militia groups in Basra on March 25, launched by Nuri al-Maliki, the prime minister, was aimed at cracking down on armed groups and strengethening government-backed forces.
However, al-Sadr maintains that Mahdi Army fighters are being unfailry targeted, despite the efforts he believes that the group has made in trying to restore a sense of stability ot the country.
US and British forces gave reconnaissance and tactical support to the Iraqi military during the crackdown, which triggered clashes across Shia areas of Iraq, including Sadr City, al-Sadr's stronghold.
Although al-Sadr called his Mahdi Army fighters off the streets of Basra soon after the violence, raids by government forces have continued.
Hundreds of people have been killed and wounded since the operation.
Al-Sadr withdrew from public view in 2008, in part to study to become a religious authority like his ancestors. He says, however, that he maintains control of the group through a ruling committee.
Politicians loyal to al-Sadr form a 30-member bloc in the Iraqi parliament.
The Mahdi Army and its leader have been branded by the US as one of the biggest threats in Iraq, and whether al-Maliki will be able to subdue the group remains to be seen