In Baghdad, aides to al-Sadr urged followers in Sadr City to travel to Najaf to reinforce the militia. Al-Sadr's representative in the southern city of Basra, Sheik Abdul-Sattar al-Bahadli, said he would form suicide squads to carry out attacks on coalition forces and urged residents to register for the squads starting Saturday.

And in the southern city of Amarah, al-Sadr aide Farqad al-Mousawi warned Iraqi police and civil defense corps members that they risked assassination if they helped U.S. soldiers fight al-Sadr's militia.

Japan's Kyodo News service reported shooting and an explosion late Friday in Samawah, a southern city where Japanese and Dutch troops are based. The shooting started after armed, masked al-Sadr supporters began sealing off downtown streets. One Iraqi security officer was killed, Kyodo said.

Al-Sadr launched an uprising against the coalition last month after U.S. officials announced he was wanted for the April 2003 murder of a cleric in Najaf. He lacks the spiritual stature of Grand Ayatollah Ali al-Husseini al-Sistani, and his confrontational tactics have exasperated moderate Shiites.

However, al-Sadr commands the support of thousands of mainly poor, urban Shiites who admire his father, a grand ayatollah who was killed by Saddam Hussein's agents. Al-Sadr has also capitalized on hostility toward the coalition following revelations of abuse of Iraqi prisoners by U.S. soldiers.

Despite the fighting, al-Sadr delivered a sermon at Friday prayers in Kufa, another holy city that lies six miles to the northeast of Najaf, as he has for the past four Fridays.

Al-Sadr described President Bush and British Prime Minister Tony Blair as "the heads of tyranny" and accused them of ignoring the suffering of Iraqis in coalition prisons while drawing attention to what he described as the "fabricated" case of Nicholas Berg, an American civilian who was beheaded by militants.

In other developments:

• The U.S. Army announced criminal charges, including adultery, maltreatment of detainees and committing indecent acts, against Military Police Cpl. Charles A. Graner in the Abu Ghraib prisoner abuse scandal. He will be arraigned May 20. Three other military police face charges in the scandal.

• The United States freed 293 Iraqi detainees from Abu Ghraib. The coalition periodically releases detainees from the prison, which now holds more than 3,000 prisoners.

• A U.S. military supply convoy was attacked 25 miles north of Baghdad and one fuel tanker was destroyed. Iraqi youths danced and cheered around the burning vehicle as they displayed family photos, presumably the driver's.

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Contributing: Hamza Hendawi