BAGHDAD, Iraq -- The U.S. closure of the weekly newspaper of a zealously anti-American Shiite cleric has invigorated the movement and its opposition to the American-led occupation.

With the United States planning to surrender political power to Iraqis by the end of June, Washington can little afford a new front in the increasingly violent battle to pacify the country. But closing the newspaper, Al-Hawza, seems to have opened one.

Cleric Muqtada al-Sadr's supporters are well-organized and led by young, motivated clerics whose respect for their 30-year-old leader arises largely from the reverence accorded his late father, a senior cleric gunned down in 1999 by suspected agents of Saddam Hussein's regime.

The movement's social services and appeal as a powerful forum for Shiites in Baghdad's poor neighborhoods and Iraq's southern cities have generated discipline and loyalty among supporters.

Al-Hawza was closed March 28 for allegedly inciting violence against coalition troops. Al-Sadr's supporters then held huge demonstrations outside the Baghdad headquarters of the U.S.-led coalition Wednesday and Friday.

Movement leaders say if the decision is not rescinded soon, they will disrupt life in Baghdad with an indefinite citywide strike. On Saturday, thousands of black-clad militiamen loyal to al-Sadr marched in military step in Baghdad.

"It's not just a question of closing down Al-Hawza," said Abbas al-Robai, the paper's editor and a close aide to al-Sadr. "If we don't resist by all means now, they'll close our offices and ban our Friday prayers."

Also Saturday, about 3,000 people demonstrated in the holy city of Najaf, south of Baghdad, to protest the earlier detention of Mustafa al-Yacoubi, a senior al-Sadr aide. Other movement officials reported the arrest, but the U.S. military could not confirm his detention.

Later, a report circulated in Baghdad that al-Sadr's Najaf home was surrounded by coalition troops, prompting thousands of supporters to march on coalition headquarters. U.S. soldiers backed by tanks blocked their path, witnesses said.

The U.S.-led occupation authorities would be stretched thin if faced with a prolonged campaign of agitation by al-Sadr supporters. The coalition counts on the goodwill of Shiites, who comprise about 60 percent of Iraq's 25 million people, to counterbalance a tenuous security situation stemming from a deadly insurgency by Sunni Muslims north and west of the capital.

With about 600 U.S. soldiers killed since the war began a year ago, Washington would rather see the firebrand al-Sadr and his zealous supporters return to the sidelines while troops try to pacify Iraq's Sunni heartland.

Al-Sadr and his top aides have refrained from condoning or calling for armed attacks against U.S. soldiers, although frequent hints have been made in recent days.

"The enemies of God and your enemies will not be able to stand up to you," Qais al-Khazali, who deputized for al-Sadr at the militia parade, told participants. "This is the first step in defending your faith, your nation and your holy shrines."

Al-Sadr's group was prominent in the months after Saddam's ouster but lately had faded from the limelight, largely because of the rise of Grand Ayatollah Ali al-Husseini al-Sistani as the Shiites' top cleric.

The Iranian-born Al-Sistani augmented his standing among Shiites when he twice forced Washington to drop political programs for Iraq that he objected to.

Ostensibly, Saturday's parade was staged as a prelude to the militia's expected role in maintaining law and order when hundreds of thousands of pilgrims flock to holy Shrines in Baghdad and the southern city of Karbala on a major Shiite feast next week.

But the timing suggests it was meant to remind the coalition of the movement's strength. The parade, held at the mainly Shiite district of Sadr City, followed al-Sadr's expression of solidarity with the Palestinian militant group Hamas in a Friday sermon.

"Let (Hamas) consider me their striking arm in Iraq because the fate of Iraq and Palestine is the same," said the charismatic, politically savvy cleric.

Hamas has killed hundreds of Israelis in suicide bombings.

Many of the parade's participants wore ski masks inscribed with the word "al-Shahadah," or "martyrdom." Some chanted "No, no America!," while others carried swords used by Shiites to perform rituals of self-flagellation.

Participants were unarmed, but scores of security men carried rifles and pistols.