Hadi Mizban, Associated Press
An Iraqi army soldier stands guard at sunset on the first day of combat operations in Amarah.

BAGHDAD — Iraqi troops arrested the mayor of the southern city of Amarah on Thursday, raising tensions with followers of anti-U.S. cleric Muqtada al-Sadr as a third Iraqi military operation against Shiite militias in recent months got under way.

U.S.-backed Iraqi soldiers and police fanned out across the city of about 450,000, a Sadrist stronghold and hub of smuggler networks bringing in weapons from Iran to Shiite extremists. There was no resistance, and some gunmen avoided arrest by tossing weapons into the streets or irrigation canals.

The operation is part of a campaign by Prime Minister Nouri al-Maliki to assert government control in areas of the Shiite south where militias have held sway for years.

Mayor Rafia Abdul-Jabbar, who also serves as acting deputy governor of Maysan province, was taken into custody at his office along with a member of the provincial council, Iraqi officials said.

Abdul-Jabbar, a Sadrist, was among 17 wanted people detained on the first day of the operation for alleged involvement in militia activities, Iraqi Defense Ministry spokesman Maj. Gen. Mohammed al-Askari told AP Television News.

Al-Sadr's aides complained that the mayor's arrest violated the spirit of agreements made in talks with the government in the run-up to the operation in Amarah, about 200 miles southeast of Baghdad. Sadrists had promised to cooperate with the operation so long as Iraqi troops did not make arrests without warrants or commit other human rights violations.

"We were surprised by the violations and the random raids in Amarah," said Hazim al-Araji, a senior al-Sadr aide in the Shiite holy city of Najaf. "We condemn the latest events that show a deliberate targeting of al-Sadr's movement."

Al-Araji said he suspected some military officers running the Amarah operation were linked to rival Shiite parties and were "trying to create a crisis and a tension through these operations."

"We were so surprised to hear of the arrest of the mayor. He was very cooperative and working to make the plan a success," al-Araji said.

Sadrist officials have long complained that rival Shiite parties with close ties to the Americans have been using security operations in other Shiite cities as a pretext to weaken al-Sadr's political movement ahead of provincial elections expected this fall.

The radical cleric's Mahdi Army militia rose up and fought government troops to a standstill when they launched an operation last March in Basra. A deal mediated by Iran brought an end to the Basra fighting and allowed the Iraqi army to enter militia neighborhoods unchallenged.

A similar agreement hammered out last month between the Sadrists and Shiite religious parties ended seven weeks of fighting in Baghdad's Shiite district of Sadr City.

Despite its reputation as a smuggling center, Amarah has been among the most violence-free cities in Iraq in recent years, raising suspicions among Sadrists that their political movement was the main target.

The military action came a day after the expiration of a four-day deadline for militants in Amarah to surrender their arms or face arrest.

In recent weeks, the Sadrists have opted against armed confrontations with Iraqi soldiers and police, apparently to avoid a punishing battle with U.S. and Iraqi forces that would threaten their political base in southern Iraq.

The Sadrists boycotted the last provincial elections in 2005 and are eager to gain seats on provincial councils, which wield considerable power in the oil-rich region. The Shiite-led government is threatening to ban parties that maintain militias, such as the Mahdi Army.

Al-Maliki, a Shiite, has vowed to destroy the power of Shiite militias. But he has promised that members of al-Sadr's political movement who are not involved in illegal activity will not be arrested.

In announcing the start of the operation, the government said Col. Mahdi al-Assadi, the provincial police commander, imposed an indefinite curfew on parts of the city but said government offices, schools and colleges would not be affected.

Iraqi security forces already have found large weapons caches and munitions in the run-up to the offensive, it said.

About 30 gunmen handed in weapons, while others were throwing them in the streets or in canals to avoid arrest, the provincial spokesman said, speaking on condition of anonymity because he was not authorized to release the information. He added that large weapons caches were found outside the city.

In a bid to shore up support among the populace, Iraqi forces opened recruiting centers for the police and the army in the city center — offering jobs to young men in hopes of luring them away from militias.