BAGHDAD, Iraq (AP) – The Shiite militia run by the anti-American cleric Muqtada al-Sadr briefly seized control of the southern Iraqi city of Amarah on Friday in one of the boldest acts of defiance yet by the country’s powerful, unofficial armies.

The militiamen later withdrew from the streets after Iraqi soldiers and mediators arrived, lifting their siege of police headquarters under a temporary truce negotiated with an al-Sadr envoy. It was not clear whether the cleric knew about his militia’s planned takeover in advance.

British military spokesman Maj. Charlie Burbridge said 600 Iraqi army soldiers had retaken control of the city, but not before the 25 gunmen and police were killed in violence that began Thursday night. The Iraqi army dispatched two companies to Amarah, a city of 750,000, from Basra, the south’s largest city.

“They’ve applied a solution and at the moment it’s holding,” Major Burbridge said. “At the moment, it’s tense but calm,” he said.

Britain had 500 soldiers on standby if called for, Major Burbridge said, saying British military authorities were “confident that they’ve (Iraqi security forces) responded as best as they can.”

Mahdi Army fighters had stormed three main police stations Friday morning, residents said, planting explosives that flattened the buildings in Amarah, a city just 30 miles from the Iranian border that was under British command until August, when it was returned to Iraqi government control.

About 800 black-clad militiamen with Kalashnikov rifles and rocket-propelled grenades were patrolling in commandeered police vehicles, witnesses said. Other fighters set up roadblocks on routes into the city and sound trucks circulated telling residents to stay indoors.

AP Television News footage showed thick, black smoke billowing from behind the barricades of a police station in Amarah. Much of the smoke came from fires set to vehicles that were parked within the compound. Hooded gunmen roamed the streets, some of whom seemed to be directing the others, while a stream of gunshots could be heard in the background.

Some streets were entirely deserted except for the gunmen, but on others children ran around, pointing out the source of gunfire, and a couple of bicyclists stopped to look at the smoke that enveloped the police station.

The events in Amarah – involving a dispute between the Mahdi Army and local security forces believed controlled by the rival Badr Brigade militia – highlight the threat of wider violence between rival Shiite factions, who have entrenched themselves among the majority Shiite population and are blamed for killings of rival Sunnis.

Shiite militia violence, mainly against the country’s Sunni minority, has ravaged Iraq since February when a Shiite holy place in Samara was blown up. The violence has been on the increase, but this is the first recent fighting that has pitted Shiites against one another on such a scale.

Prime Minister al-Maliki, a former Shiite activist, won the top government post last spring thanks in part to the support of Mr. al-Sadr, who controls 30 of the 275 seats in the national parliament and five Cabinet posts.

In a sign of Mr. al-Sadr’s influence, Mr. al-Maliki this week ordered the release of one of the young cleric’s top lieutenants, Sheik Mazen al-Sa’edi, who was arrested by American troops in Baghdad for alleged links to sectarian death squads.

The fighting in Amarah came just days after Mr. al-Maliki met with Mr. al-Sadr at the cleric’s headquarters in the holy city of Najaf to enlist support for reining in sectarian violence and building political stability.

The timing seemed to indicate Mr. al-Sadr and other Mahdi Army commanders did not have full control over individual units, lending weight to the theory that many such militia groups were acting on their own and carrying out local agendas.

Mahdi Army militiamen have long enjoyed a free rein in Amarah, the provincial capital of the southern province of Maysan. Militiamen in Amarah often summon local government officials for meetings at their offices. They roam the city with their weapons, manipulate the local police and set up checkpoints at will.

Since British troops left Amarah in August, residents say the militia has been involved in a series of killings, including slayings of merchants suspected of selling alcohol and women alleged to have engaged in behavior deemed immoral by militiamen.

Fighting broke out Thursday after Qassim al-Tamimi, the provincial head of police intelligence and a leading member of the rival Shiite Badr Brigade militia, was killed by a roadside bomb. In retaliation, his family kidnapped the teenage brother of the Mahdi Army commander in Amarah, Sheik Fadel al-Bahadli, to demand the hand-over of al-Tamimi’s killers.

Amarah, a major population center in the resource-rich yet impoverished south, is a traditional center of Shiite defiance to successive Iraqi regimes. Its famed marshlands were drained by former dictator Saddam Hussein during the 1990s in reprisal for the city’s role in the Shiite uprising that blazed through the region after the 1991 Gulf War.

The showdown between the Mahdi and Badr militias has the potential to develop into an all-out conflict between the heavily armed groups and their political sponsors, both with large blocs in parliament and backers of al-Maliki’s ruling coalition. It also could shatter the unity of Iraq’s majority Shiites at a time when an enduring Sunni insurgency shows no signs of abating.

The U.N. refugee agency said at least 914,000 Iraqis have fled their homes since the U.S.-led invasion in 2003, more than a third in response to the sectarian bloodshed this year.