BAGHDAD, Iraq -- The top U.S. administrator in Iraq declared a radical Shiite cleric an "outlaw" Monday after his supporters rioted in Baghdad and four other cities in fighting that killed at least 52 Iraqis, eight U.S. troops and a Salvadoran soldier.

The fiercest battle took place Sunday in the streets of Sadr City, Baghdad's largest Shiite neighborhood, where black-garbed Shiite militiamen fired from rooftops and behind buildings at U.S. troops, killing the eight Americans. At least 30 Iraqis were killed and more than 110 wounded in the fighting, doctors said.

Violence broke out Monday morning in another Shiite neighborhood of the capital, al-Shula, where followers of the cleric clashed with a U.S. patrol. A U.S. armored vehicle was seen burning, and an Iraqi man was seen running off with a heavy machine gun apparently taken from the vehicle. A U.S. helicopter hovered overhead. There were no immediate reports of injuries.

Hundreds were wounded in Sunday's violence in Baghdad, Najaf, Nasiriyah and Amarah. The riots were ignited by the arrest of an aide to anti-American cleric Muqtada al-Sadr. Followers of al-Sadr also took over the offices of the governor in the southern city of Basra.

"Effectively he is attempting to establish his authority in the place of the legitimate authority. We will not tolerate this. We will reassert the law and order which the Iraqi people expect," Bremer told a security team meeting convened to discuss how to respond to Sadr.

Bremer did not say whether U.S. authorities would move to arrest al-Sadr, but the declaration stepped up the confrontation with the 30-year-old firebrand cleric, who fiercely opposes the U.S. occupation and is backed by a militia known as the "Al-Mahdi Army."

"There is no room for militias in the new Iraq. ... If there are militias that seek to exert control, we will address that head on. And that is clearly what we are doing right now," a senior U.S. official said Monday.

Al-Sadr does not hold widespread support among Iraq's Shiites, many of whom see him as too young, radical and inexperienced to lead. But he does have the backing of hundreds of young seminary students and many impoverished Shiites, devoted to him because of his anti-U.S. stance and the memory of his father, a Shiite religious leader gunned down by suspected agents of Saddam Hussein in 1999.

Al-Sadr has demanded an immediate U.S. withdrawal from Iraq, and his followers have protested against U.S.-backed local officials in several towns in the south in previous months. But the cleric's political program has often been unclear.

The violence Sunday was a sign of al-Sadr's strength.

The U.S. troops moved into Baghdad's Sadr City -- named after Muqtada's father -- after militiamen ambushed a U.S. patrol in the neighborhood, said Major General Martin Dempsey, commander of the Army's 1st Armored Division.

More troops streamed into the neighborhood -- up to 1,000 at one point -- fighting sporadic gun battles with more than 500 militiamen, Dempsey said. The fighting ended after a column of tanks moved in.