Supporters of the militant Shiite cleric Moqtada Sadr challenged authorities in Baghdad on Tuesday by setting up makeshift checkpoints and attacking police stations in a bid to widen a confrontation centered in the southern city of Najaf. An official at the Health Ministry said 10 people were killed here and more than 100 wounded.

Gunmen briefly asserted control of some Baghdad neighborhoods and called for a curfew over the entire city. Authorities rejected the demand and said the city remained securely under government control, despite scattered reports that some policemen had chosen to hide rather than fight.

Residents of several neighborhoods said streets emptied when members of the Mahdi Army, the militia loyal to Sadr, came through, apparently unchallenged by the police.

"They send small groups to the streets to set up checkpoints and terrify people," said a police official in Zayouna, an upper-class neighborhood of Baghdad, speaking on condition of anonymity. "It is difficult to fight them in the residential neighborhoods and narrow alleys because the Mahdi Army controls these places."

But Sabah Kadhim, a spokesman for the Interior Ministry, dismissed the accounts of roving Mahdi Army patrols as "rumors and lies. They are cowards, and they use a hit-and-run method to create chaos to confuse the police. We will not give them a chance to control the streets."

The Mahdi Army appeared to control Sadr City, a Shiite stronghold in northeast Baghdad, flouting a government-imposed curfew in the area. U.S. troops remain largely outside Sadr City, and Iraqi police and security forces have not challenged Sadr's forces there. A resident of Sadr City said Tuesday that it was quiet there until evening, when clashes resumed.

Heavy clashes were reported in Baghdad's Mansour district, and there were numerous mortar strikes in Baghdad on Tuesday morning, many of them targeting police stations and government buildings. The ministries of oil and information both were struck, but they sustained minimal damage.

Meanwhile, kidnappers released a Lebanese businessman taken hostage last month. But a video on a Muslim Web site purported to show the beheading of an Egyptian man who confessed on the tape to working "as a spy for the Americans in Iraq." The authenticity of the video could not be immediately established.

In neighboring Iran, the government announced that it had invited Prime Minister Ayad Allawi to visit, an apparent attempt to defuse the growing antagonism between the two countries, which fought a devastating war from 1980 to 1988. Iraqi authorities have accused Iran of sending fighters, spies and weapons to Iraq, an allegation that Iran has denied.

In another development, the U.S. military said pretrial hearings for four soldiers accused of abusing detainees at Abu Ghraib prison would be held in Germany this month.

The two-day hearings for Staff Sgt. Ivan L. "Chip" Frederick II, Sgt. Javal S. Davis, Spec. Charles A. Graner Jr. and Spec. Megan Ambuhl are scheduled to start Aug. 23 in Mannheim. The legal procedures, known as Article 39 hearings, are equivalent to civilian court arraignments.

"The civilian defense attorneys requested that the Article 39 hearings not take place in Iraq due to the travel, the expense and the danger," said Col. Jill Morgenthaler, a spokeswoman for U.S.-led military forces in Iraq. "However, the venue for any courts-martial has not changed. They will still take place in Iraq."

Correspondent Jackie Spinner and special correspondent Luma Mousawi contributed to this report.