BAGHDAD, Iraq - The top U.S. administrator in Iraq declared a radical Shiite cleric an "outlaw" today 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 this morning in another Shiite neighborhood of the capital, al-Shula, where followers of the cleric clashed with a U.S. patrol. An American 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.

Meanwhile, hundreds of U.S. and Iraqi troops in tanks, trucks and other vehicles surrounded the turbulent city of Fallujah today ahead of a major operation against insurgents following the grisly slayings of four American security contractors last week.

Explosions and gunfire could be heard coming from the center of the city. Streets on the outskirts were largely deserted.

One U.S. Marine was killed in fighting in the area today, said Lt. Col. Gregg Olson, commander of the 2nd Battalion, 1st Marine Regiment.

U.S. commanders have been vowing a massive response to pacify Fallujah, one of the most violent cities in the Sunni Triangle, the heartland of the anti-U.S. insurgency north and west of Baghdad.

After the slayings of the Americans on Wednesday, residents dragged the four bodies through the streets, hanging two of their charred corpses from a bridge, in horrifying scenes that showed the depth of anti-U.S. sentiment in the city.

U.S. troops closed off entrances to Fallujah with earth barricades ahead of the planned operation, code named "Vigilant Resolve." Military patrols entered the outer suburbs on reconnaissance missions and to broadcast warnings on loud speakers to residents to stay indoors until Tuesday.

Iraqi police in the city visited mosques, dropping off Arabic leaflets from the U.S. military, telling residents that there was a daily 7 p.m. to 6 a.m. curfew. It ordered them not to congregate in groups or carry weapons, even if licensed. It instructed people that if U.S. forces enter their homes, they should gather in one room and if they want to talk to the troops to have their hands up.

Some 1,200 U.S. Marines and two battalions of Iraqi security forces were poised to enter the city to arrest suspected insurgents, said Lt. James Vanzant, 2nd Battalion, 1st Marine Expeditionary Force.

"The city is surrounded," Vanzant said. "It's an extended operation. We want to make a very precise approach to this. ... We are looking for the bad guys in town."

Marine 1st Lt. Eric Knapp said the troops will target the killers of the four Americans, as well as rebels who have attacked U.S. forces and Iraqi police in the past month. "Those people are specially targeted to be captured or killed," he said.

A Marine officer, speaking on condition of anonymity, said U.S. forces had a list of targets for raids.

A witness reported that a U.S. helicopter struck a residential area in the city early this morning, killing five people. The bombing damaged five houses, said the witness, Mohammed Shawkat. There was no immediate U.S. comment on the report.

Another witness, resident Ali Jasim, said there was shooting near one of the U.S. barricades on a road out of Fallujah and some Iraqis who were trying to leave the city were hit. It was unclear whether they were killed or wounded. Roads to a hospital in Fallujah were blocked to all traffic except ambulances.

On Sunday, a suicide attacker detonated a bomb-laden vehicle as he tried to enter a U.S. base in the northern city of Kirkuk, killing one U.S. soldier and wounding six Americans and six Iraqis, the military said. A roadside bomb killed a U.S. soldier in the city of Mosul.

The violence pushed the U.S. death toll in Iraq to at least 613.

The insurgency that has plagued U.S. troops in Iraq for months has been led by Sunni Muslims. But Sunday's clashes in Baghdad and three other cities threatened to open a dangerous new front: a confrontation with Iraq's powerful Shiite Muslim majority, which has until now largely avoided violence with the Americans.

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.

In response, L. Paul Bremer, the top U.S. administrator in Iraq, declared al-Sadr an "outlaw" who threatens Iraq's security.

"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 if 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 today.

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 Saddam agents 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 Maj. Gen. 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 gunbattles with more than 500 militiamen, Dempsey said. The fighting ended after a column of tanks moved in.

During the fighting, police evacuated three stations to regroup in a single station, and Mahdi Army militiamen seized the abandoned sites, U.S. officials said.

Al-Chawadir hospital, one of two hospitals in Sadr City, received 28 dead Iraqis and 90 wounded from the fighting, said a doctor, Qassim Saddam. Another hospital reported two dead and 21 wounded.

This morning, U.S. tanks were parked in one of the neighborhood's main markets. Many stalls in the market were burned out, and buildings were pockmarked with bullet holes.

During a street protest by some 5,000 people Sunday near the Shiite holy city of Najaf, al-Sadr supporters opened fire on the base of Spanish troops, sparking a battle that lasted several hours.

A Salvadoran soldier was killed and at least nine other soldiers were wounded, the Spanish Defense Ministry said. The U.S. military denied a report that an American soldier was killed. Twenty-two Iraqis died and more than 200 were wounded, said Falah Mohammed, director of the Najaf health department.

Al-Sadr issued a statement later Sunday calling off street protests, but he also called on followers to "do what you see fit in your provinces. Strike terror in the heart of your enemy ... We can no longer be silent in the face of their abuses."

Some of al-Sadr's followers in Baghdad said they interpreted this as a call for armed resistance against U.S. forces.

The violence was touched off by the arrest of Mustafa al-Yacoubi, a senior aide to al-Sadr, on charges of murdering Abdel-Majid al-Khoei, a rival Shiite cleric. A total of 25 arrest warrants have been issued in the case, and 13 suspects have been taken into custody, an official at the coalition headquarters said.

Al-Sadr supporters also were angered by the March 28 closure of his weekly newspaper by U.S. officials. The Americans alleged the newspaper was inciting violence against coalition troops.

In other developments today, U.N. envoy Lakhdar Brahimi met with members of Iraq's Governing Council as he launched a mission to help in the transition to an interim government after sovereignty is handed back to Iraqis on June 30.