BAGHDAD, Iraq — The Shiite militia run by anti-American cleric Muqtada al-Sadr seized total control of the southern Iraqi city of Amarah today in one of the boldest acts of defiance yet by one of the country's powerful, unofficial armies, witnesses and police said.

Prime Minister Nouri al-Maliki dispatched an emergency security delegation that included the Minister of State for Security Affairs and top officials from the Interior and Defense ministries, Yassin Majid, the prime minister's media adviser, told The Associated Press.

The Mahdi Army fighters stormed three main police stations this morning, planting explosives that flattened the buildings, residents said.

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

Fighting broke out in Amara on Thursday after the head of police intelligence in the surrounding province, a member of the rival Shiite Badr Brigade militia, was killed by a roadside bomb, prompting his family to kidnap the teenage brother of the local head of the a-Madhi Army.

The Mahdi Army seized several police stations and clamped a curfew on the city in retaliation.

At least 15 people, including five militiamen, one policeman and two bystanders, have been killed in clashes since this morning, Dr. Zamil Shia, director of Amarah's department of health, said by telephone from Amarah.

The events in the city highlight the threat of wider violence between rival Shiite factions.