Violence broke out Monday in a second Baghdad Shiite neighborhood, al-Shoala, as members of radical Shiite cleric Muqtada al-Sadr's "Al-Mahdi Army" militia clashed with a U.S. patrol. An American armored vehicle was seen burning and a U.S. helicopter hovered overhead.

Militiamen also clashed with British troops in two southern cities, and three Iraqis were killed, witnesses said, while in several cities al-Sadr followers held sit-in protests, controlled the streets and held police stations.

A Marine was killed Monday in the Fallujah (search) area, the military said, without providing further details. The death — along with killings of U.S. soldiers in two northern cities and in Sunday's violence — brought the U.S. death toll in Iraq to at least 613 since the war on Iraq began a year ago.

Violent clashes Sunday between militiamen loyal to al-Sadr killed 52 Iraqis, eight U.S. soldiers and a Salvadoran soldier, who some witnesses said was forced to eat a live grenade. It was some of the worst violence since the ouster of Saddam Hussein.

American civilian administrator L. Paul Bremer (search) on Monday declared al-Sadr an "outlaw." Later, an Iraqi judge issued an arrest warrant for him for the slaying of another Shiite leader shortly after the U.S.-led invasion of the country, coalition officials said.

U.S. commanders have been vowing a massive response after insurgents killed four American security contractors in the Sunni city of Fallujah, west of Baghdad, Wednesday.

Residents dragged the Americans' bodies through the streets, hanging two charred corpses from a bridge in horrifying scenes that showed the depth of anti-U.S. sentiment in the city.

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.

Al-Sadr fiercely opposes the U.S. occupation, but he has not had widespread support among Shiites, who see him as too young, too radical and too inexperienced to lead.

The large protests Sunday in Baghdad, Najaf, Nasiriyah and Amarah — sparked by the arrest of an aide to al-Sadr — were a show of force by his militia. But U.S. officials said they believe his appeal among the Shiite public remained limited.

Mustafa al-Yacoubi (search), a senior aide to al-Sadr, was arrested last week on charges of murdering Abdel-Majid al-Khoei (search), a rival Shiite cleric.

A total of 25 arrest warrants have been issued in the case, possibly including the one for al-Sadr himself, and 13 suspects have been taken into custody, an official at the coalition headquarters said.

The al-Mahdi Army was out in force Monday in several southern Iraqi cities. In Basra, Shiites who seized the governor's office traded fire with British troops. One Iraqi was killed, al-Sadr's office in the city said.

In the nearby city of Amarah, militiamen marched in the streets, clashing with British troops near the governor's office. Two Iraqis were killed in the exchange of the fire, witnesses said.

Gunmen also held sway in the holy city of Najaf, prompting police to flee their stations, said the Spanish Defense Ministry, whose troops control the region. Witnesses said the police returned later.

The Spanish bases in Diwaniya and Najaf came under sporadic mortar fire overnight but there were no injuries or material damage, the ministry said.

Al-Sadr's main support is among 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, but the cleric's political program has often been unclear. He has repeatedly said that he does not seek political power, but wants an "honest and responsible" government.

The latest violence suggests that al-Sadr wants to raise the profile of his movement at a time when other ethnic and religious groups in Iraq are jockeying for position ahead of the transfer of power to Iraqis.

Sunday evening, U.S. troops moved into Sadr City (search) — 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 in — 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, which Mahdi Army militiamen then seized, U.S. officials said. They were later forced out by U.S. troops, they added.

Monday 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.

The other main fighting Sunday came outside Najaf, where militiamen opened fire on the base of Spanish troops, sparking a battle that lasted several hours. A Salvadoran soldier and 22 Iraqis were killed, Spanish and Iraqi officials said.

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.

Al-Sadr supporters also were angered by the closure of his weekly newspaper by U.S. officials, who accused it of inciting violence.

In other developments:

— U.N. envoy Lakhdar Brahimi (search) 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.

— 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, meanwhile, killed a U.S. soldier in the city of Mosul.