Updated

U.S. administrators in Iraq declared a radical Shiite (search ) cleric an "outlaw" Monday and announced a warrant for his arrest, heightening a confrontation after battles between his supporters and coalition troops killed at least 52 Iraqis and nine coalition troops, including eight Americans.

American officials would not say when they would move to arrest Muqtada al-Sadr (search ), who is holed up in the main mosque in Kufa, south of Baghdad, guarded by armed supporters.

"We don't fear death and martyrdom gives us dignity from God," said al-Sadr, a 30-year-old firebrand who has frequently denounced the U.S. occupation in his sermons.

The Americans "have the money, weapons and huge numbers, but these things are not going to weaken our will because God is with us," he said in a statement sent to the Arab TV station Al-Jazeera, which provided a copy to The Associated Press.

L. Paul Bremer (search ), the top U.S. official in Iraq, cancelled a trip to Washington this week, a Senate aide said Monday. The aide said Bremer was to have given a closed-door briefing Thursday to the full Senate on the situation in Iraq, but Senate officials were informed Monday morning that the visit to Washington had been put off. No reason was given for the postponement, the aide said.

U.S. troops surrounded the city of Fallujah (search ), west of Baghdad, poised for a major operation in response to the grisly slaying and mutilation of four American civilians by insurgents there last week. A Marine was killed Monday in the Fallujah area, the military said, without providing details.

The showdown with al-Sadr threatened to heighten tensions with Iraq's Shiite Muslim majority at a time when U.S. troops are burdened by the Sunni guerrillas' bloody insurgency. But American officials apparently hope the Shiite public -- many of whom distrust al-Sadr -- will not rally around the cleric.

Brig. Gen. Mark Kimmitt said the potential for violence depended on "whether [al-Sadr] decides to come peacefully or whether he decides to come not peacefully. That choice is the choice of Mr. Muqtada al-Sadr."

Several hundred of al-Sadr's armed militiamen control Kufa, holding its police station and blocking a road leading to the main mosque.

Sheik Abu Mahdi al-Rubaie, a 35-year-old al-Sadr follower at the mosque, warned that any U.S. move against al-Sadr would be "a very dangerous thing."

"They will pay a heavy price. We will not allow them to enter Kufa ... We are ready to lay down our lives for al-Sayed," he said, using the Arabic word for "master" to refer to al-Sadr.

A senior officer in Washington said U.S. military commanders have begun studying ways they might increase troops in Iraq should violence spread much more widely.

Generals believe they have enough forces to handle the attacks, including the Shiite militia violence, but want to know what is available if the situation gets worse, said the officer, briefing reporters on condition of anonymity.

U.S. officials said the warrant against al-Sadr -- on charges of murdering a rival cleric -- was issued months ago by an Iraqi judge and that Iraqis only now want to carry it out. The crackdown on the opponent of the U.S. administration also comes as the June 30 deadline approaches for the transfer of power from the Americans to the Iraqis.

President Bush on Monday portrayed al-Sadr's removal as a step toward protecting democracy. "This is one person that is deciding that rather than allowing democracy to flourish, he's going to exercise force," he told reporters. "We just can't let it stand."

Bremer declared al-Sadr an "outlaw."

"He is attempting to establish his authority in the place of the legitimate authority. We will not tolerate this," Bremer said.

Sunday's clashes -- sparked by the arrest of an al-Sadr aide who is also accused in the slaying of rival cleric Abdel-Majid al-Khoei -- were a surprise show of power by al-Sadr's militia, the Al-Mahdi Army.

Fighting was particularly fierce in Sadr City, a Shiite-majority neighborhood in Baghdad, in what Maj. Gen. Martin Dempsey, commander of the U.S. forces in Baghdad, called the biggest gunbattle since the fall of Baghdad a year ago. Eight U.S. soldiers and at least 30 Iraqis were killed.

Some 1,000 U.S. troops moving into the area at one point for the battle with hundreds of militiamen using everything from automatic weapons and rocket-propelled grenades to homemade pipe bombs.

It began when gunmen ambushed a U.S. patrol, forcing the Americans to take refuge in a nearby building and call in reinforcements, Dempsey told reporters.

The Humvees and armored vehicles that came in to help came under three separate ambushes. "It was a very calculated action" by the militiamen, Dempsey said. "It's really a mob, a mob with a lot of weapons."

Gunmen sprayed weapons fire from rooftops and fired RPGs from alleyways. Militiamen seized three police stations after police evacuated and moved to a single station for safety. More than four hours of fighting ended and the stations were retaken only after a column of tanks rumbled into the neighborhood in the evening, Dempsey told reporters.

Outside the city of Najaf, south of Baghdad, firing between militiamen and Spanish-led coalition troops killed one Salvadoran soldier and 22 Iraqis on Sunday.

Violence broke out Monday morning in another Shiite neighborhood of the capital, al-Shoala, where militiamen clashed with a U.S. patrol. An American armored vehicle caught fire, and an Iraqi ran away with a heavy machine gun. A U.S. Apache helicopter was hit by small arms fire and responded with a barrage of machine-gun rounds, the U.S. military said.

Militiamen also traded fire with British troops in the southern cities of Basra and Amarah, sparking fights that killed three Iraqis, witnesses said.

Gunmen also held sway in the streets of 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 later returned.

The Spanish bases in Diwaniyah and Najaf came under sporadic mortar fire overnight Sunday but there were no injuries, the ministry said.

Al-Sadr's main support is among young seminary students and impoverished Shiites, devoted to him because of his anti-U.S. stance and the memory of his father, a religious leader gunned down by suspected agents of Saddam Hussein in 1999.

However, al-Sadr's religious status is low, giving him less influence than more moderate Shiite leaders. And many Shiites see him as erratic.

The arrest warrant against al-Sadr is on charges of involvement in the April 2003 murder of al-Khoei, who was stabbed to death by a mob in a Shiite shrine in Najaf soon after Saddam's fall, said coalition spokesman Dan Senor.

Sunday's violence was touched off by the arrest of Mustafa al-Yacoubi, a senior aide to al-Sadr, on similar charges. A total of 25 arrest warrants have been issued in the case, and 13 suspects are in custody, an official at coalition headquarters said.

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

In other developments:

-- U.N. envoy Lakhdar Brahimi met with members of Iraq's Governing Council as he launched a mission to help with the transition to an interim government after sovereignty is transferred to the Iraqis on June 30.

-- A homicide 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.

At least 613 U.S. troops have been killed in Iraq since the war began a year ago.