"This army is helping society," Bakr said. "They clean the streets, protect our schools and distribute fuel and gas."
It has been estimated by the Iraq Study Group, from the US Institute of Peace, that the force consists of 60,000 fighters.
But some Iraqis say there are more and that they are present in every city and town from Baghdad to the southern border with Kuwait.
"One cannot believe that the Shia-Sunni civil war was not pre-planned by the West"
Mohammad Karim, Melbourne, Australia
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Al-Sadr is against the US occupation of Iraq and has been demanding a timetable for US withdrawal.
In Sadr City, the group's stronghold, the Mahdi army forbids black markets, which are rampant in the rest of the capital, and mans strict checkpoints to detect car bombs.
"Ask anyone around," says one of its foot soldiers, "they will tell you that without our presence, they will not be able to sleep at night, students will not be able to go to school, like in the rest of the capital where people are scared."
Many Sunnis however are petrified of the group which they accuse of carrying a relentless ethnic cleansing campaign against them.
Everyday, bodies are found dumped in and around the capital, most showing signs of torture.
Abdullah is a Sunni student from Baghdad. He says that what scares him most is the Mahdi army.
"If anyone from them [the Mahdi army] recognised that I am Sunni, then I will be targeted," he said.
Like many Sunnis, Abdullah carries different identification papers with different names "to be on the safe side".
The group is accused of infiltrating the security forces. Its members use police uniforms and set up fake checkpoints to hunt down Sunnis.
They have seized control of some neighbourhoods of the capital and chased Sunnis away.
|Muqtada al-Sadr [EPA]|
Baghdad is now a city divided along sectarian lines. Abdullah said he is living "as if chained to one area".
The Mahdi army is a direct product of the US occupation in Iraq.
It has concentrated on fighting US troops, and on two occasions sent aid to Sunnis in Fallujah during military offensives led by US forces.
That support stopped in February 2005, when the al-Askari shrine, a holy site for Shia Muslims in Samarra, was bombed.
"It changed the situation," Abdullah said.
"We were once friends and brothers, living together peacefully … It was as if something was planned in order to stimulate sectarian violence."
Within hours of the bombing, young people were riding around the capital on the back of pick-up trucks, parading guns and vowing for revenge.
Nuri Al-Maliki, the Iraqi prime minister, has been reluctant to confront the Mahdi army, ordering the release of the group's leaders and lifting a US blockade over their Baghdad stronghold in October.
Muqtada al-Sadr and the Mahdi army have been branded by the US as one of the biggest threats in Iraq.
US forces have arrested Abdel-Hadi al Daraji, spokesman of the al-Sadr movement, accusing him of being an operational leader of death squads throughout Baghdad.
"Sectarianism is like planting a seed. As it grows, it carries on with people, from generation to generation"
Abdullah, Iraqi student
Iraqi officials say that anyone carrying a weapon will be targeted during the new security crackdown.
The government recently announced the arrest of 400 members of the Mahdi army and a few days later al-Sadr leaders announced that they were ending a two-month boycott of the government.
Abdullah said there have been many security crackdowns before, and none of them has achieved security.
"Sectarianism is like planting a seed. As it grows, it carries on with people, from generation to generation," he said.
"Only when another tree is planted, a tree of morality, will reconciliation between politicians and people take place. Then we will help our country."