BAGHDAD — Iraqi soldiers fired in the air over supporters of anti-U.S. Shiite cleric Muqtada al-Sadr to prevent them from gathering for Muslim prayers Friday in the southern city of Basra, enraging the worshippers and straining a fragile truce with the government.

In another worrisome sign, a top aide to al-Sadr accused Iraqi forces of violations of a separate truce in Baghdad’s Sadr City, where thousands of Iraqi troops have deployed in what has so far been a peaceful campaign to impose control.

Sadr Movement officials in both Basra and Sadr City said they were abiding by the cease-fires, but the shooting in the southern city angered al-Sadr’s followers throughout southern Iraq and in Baghdad.

The cease-fires are crucial to Iraqi security forces’ sweeps in Basra and Sadr City, launched by Prime Minister Nouri al-Maliki to show his government can spread its authority in areas long dominated by armed groups like al-Sadr’s powerful Mahdi Army militia.

Iraqi police in Basra said one person was wounded in the shooting in a square in the northern part of the city. But al-Sadr officials contended that one person was killed and three wounded.

The confrontation began when about 75 al-Sadr supporters tried to gather in Basra’s Mile-5 Square to hold Friday prayers, witnesses said. Iraqi police recently banned al-Sadr gatherings in the square after a large cache of weapons was found nearby, police officials said, speaking on condition of anonymity because they feared retaliation.

Iraqi troops were deployed at the square to prevent the prayers. When those gathering refused to disperse, the police fired rounds over their heads, witnesses and the police officials said.

“Oh al-Sadr, to whom we should complain, al-Maliki is just like Saddam Hussein,” the worshippers chanted, according to the witnesses.

In the Shiite holy city of Kufa, 210 miles northwest of Basra, a senior aide to al-Sadr, Abdul-Hadi al-Mohammadawi, denounced the incident. In a sermon, he said the security forces “opened fire on worshippers” and that one person was killed.

“Are these the benefits of the Knights’ Charge battle?” he said sarcastically, referring to the code name of the Iraqi sweep in the city.

Still, an official in al-Sadr’s office in Basra said the group would maintain the truce. “The issue of the truce is a matter of a decision from our leader Muqtada al-Sadr, not something based on this incident,” he said, speaking on condition of anonymity because he was not authorized to talk to the press.

When Iraqi forces began their sweep in Basra in late March, it sparked heavy fighting with the Mahdi Army, which also launched a wave of violence across southern Iraq and in Sadr City. The fighting in Basra and the rest of the south ended with a truce, mediated by Iran, in mid-April.

Since then, Iraqi security forces have continued raids to arrest wanted figures among the many Shiite militias in Basra, and the troops appear to have greater control of the streets in many districts. Violence has been reduced considerably.

But violence continued for weeks afterward in Sadr City until a cease-fire last week. As part of the truce, some 10,000 Iraqi soldiers and police have deployed into the district, home to 2.5 million Shiites.

An al-Sadr aide in the district, Muhannad al-Gharawi, accused some Iraqi forces of insulting “our symbols” at checkpoints.

“This is a violation of the truce,” he said without elaborating, and he said the Sadrist Movement had complained to the government.

During Friday prayers, a cleric told worshippers that the Mahdi Army would stick to the cease-fire. “If we fought them, we would risk wasting the martyrs’ blood and destroy all accomplishments of the Mahdi Army,” Sheik Muhannad al-Moussawi said.

The Sadrist Movement appears to have agreed to the truces to avert a full-scale government move to suppress the Mahdi Army, a fight that would likely cause heavy casualties and hurt al-Sadr’s longer term goal to keep his status as a major political player. Under the truce, the Mahdi Army keeps its light weapons in Sadr City.

However, Iraqi forces plan eventually to begin searching for any heavy weapons and arresting wanted figures — moves that could raise tensions and further strain the truce.

Meanwhile, in the city of Balad, 50 miles north of Baghdad, the U.S. military said more than 180 suspected Sunni Arab insurgents turned themselves in to Iraqi authorities as part of a U.S.-led effort to persuade insurgents to end their fight against American forces.

And southwest of Baghdad, an American soldier was killed Thursday by a roadside bomb, the U.S. military reported.