Iraqi volunteers train for military duty on a dusty soccer field in the town of Mahanawiyah. (Scott Nelson/For The Washington Post)

In a house on a side street in this run-down city, Saadiq al-Khakani has exchanged his religious robes for a checked shirt and pants to avoid being recognized.

The young cleric is the local representative of Mahmoud al-Sarkhi, a fringe anti-government Shiite leader whose supporters clashed last week with security forces in the holy city of Karbala, 80 miles northwest, leaving at least seven dead. Since then, more than 100 of the cleric’s followers have been rounded up in Iraq’s largely Shiite south as the government attempts to curb bubbling instability. Khakani’s brother and cousin were arrested in a recent raid on his office.

The clashes, during which security forces used helicopters to fire on Sarkhi’s compound, were the first significant instance of intra-Shiite violence since a Sunni militant group seized control of the northern city of Mosul last month in its quest to build an Islamic state. But analysts warn that more infighting is likely as Shiite militias with historical frictions restructure and rearm, creating a complex array of irregular military groups over which the state wields limited control.

Diwaniyah, 110 miles south of Baghdad, is no stranger to Shiite-on-Shiite violence. It was here that fighters from the Mahdi Army militia, loyal to radical cleric Moqtada al-Sadr, battled Iraqi and U.S. forces in 2006. Posters of the cleric still outnumber those of Prime Minister Nouri al-Maliki on the city’s streets.

Authorities suspect that Sarkhi, a former student of Sadr’s father known for his outlandish statements, could be hiding here after the Karbala clashes. They say his supporters have attacked eight police checkpoints in the past week.

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When security chiefs in Diwaniyah held an emergency meeting Thursday, the main item on the agenda was controlling Sarkhi’s followers, not the threat from the Islamic State — the al-Qaeda-
inspired Sunni group that has swept northern and western Iraq.

“The Shiites are more divided than ever,” Khakani said. “We have warned and warned that this government will bring sectarian chaos.”

He asserts that the Karbala violence began when security forces surrounded Sarkhi’s compound after the cleric spoke out against a fatwa, or religious edict, by the nation’s top Shiite cleric, Grand Ayatollah Ali Sistani, that called on Iraqis to take up arms against the Islamic State. Sarkhi believes the fatwa will lead to sectarian war, Khakani said.

Tens of thousands of Iraqis, largely Shiites, have answered Sistani’s call to fight the Sunni insurgency, including more than 4,000 from Diwaniyah, said Hussein al-Budairi, head of the city’s security committee.

But the fatwa, which emphasized the participation of civilian volunteers, also has given Iraq’s Shiite militias cover to reactivate. Iranian proxies such as Asaib Ahl al-Haq, which once carried out deadly attacks on U.S. troops and now is spearheading security operations for the Iraqi army, say they are heeding Sistani’s call.

Budairi is affiliated with the Badr Organization, a political party whose militia have taken up arms again. He claims to be “independent” and says there is no place for militias in the armed forces.

The Mahdi Army also is rearming, although under the guise of “peace brigades.” Tens of thousands of militiamen loyal to Sadr, an outspoken critic of Maliki, marched in Baghdad and cities across the south last month.

Phillip Smyth, a researcher at the University of Maryland who specializes in Shiite Islamist militarism, said the show of strength was aimed not just at the Islamic State.

“These parades are held in Shiite areas. They send a message to Maliki,” he said. “There is far more of an internal power dynamic there.”

Maliki is struggling to unite his fellow Shiites behind him, with Sadr and a major political bloc led by the cleric Ammar al-Hakim demanding that he step down.

In Diwaniyah, unemployment and poverty stir discontent. The town’s tire factory, which once distributed its wares across the country, now barely functions. Next door, the textile plant lies long shuttered. With few employment options, many young men join the Iraqi army, with the 8th Division stationed in the town.

But just 70 soldiers remain in its barracks, Budairi said. “Frankly, soldiers left because they are cowards,” he said.

The state’s use of irregular forces and militias has helped to stem the Islamic State's rampage. But as battle lines stagnate, Smyth argues, it is only a matter of time before the united front against a common enemy begins to fray.

“The less advances, then the more potential for internal fighting,” he said. “There will be a tipping point.”

For Diwaniyah residents, the proliferation of armed men in the streets is a reminder of the worst days of the 2006-2007 sectarian war.

“It’s a very bad situation,” said Husiniya Zawad, a 60-year-old widow. Two of her sons have signed up after Sistani’s call. “Our sons have no option but to fight. This is Iraq.”