U.S. forces launched the raid as thousands prepared to flood Karbala on Sunday for the beginning of a 10-day mourning period known as Ashoura. The festival marks the death of one of Shiite Islam's most sacred saints, Imam Hussein, grandson of the Prophet Muhammad.

Residents reached by telephone speculated the raid was aimed at followers of radical Shiite cleric Muqtada al-Sadr, saying helicopters flew over his main headquarters in Karbala well past nightfall.

Al-Sadr's Mahdi Army militia, which has been accused of playing a big role in sectarian killings, has been hit repeatedly in recent weeks by operations in which key commanders have been captured or killed by U.S. and Iraqi troops.

The Iraqi force that operates with the Americans against Mahdi Army fighters is believed to be the Special Operations Command division under Brig. Gen. Fadhil Birwari, a Kurd. The unit is based near the Baghdad airport, a headquarters for U.S. operations.

Elsewhere in Baghdad, Iraqi police and hospital officials said a joint U.S.-Iraqi force searched a hospital in the volatile Sunni-dominated western neighborhood of Yarmouk.

Dr. Haqi Ismail, the hospital manager, said the raid occurred at 4:30 a.m.

"They were looking for someone, they searched all the rooms and the emergency unit," he said.

American troops confiscated weapons and ID cards from police officers and security guards at the hospital after a confrontation in which a guard demanded that the soldiers deposit their weapons at the door.

"We resolved the matter within minutes and the Americans gave the Iraqi policemen their weapons and IDs cards back and now everything is OK," said Khalaf, the Interior Ministry spokesman.

Al-Sadr's followers voiced increasing anger over Friday's capture of a senior aide to the radical cleric in a raid in eastern Baghdad.

Nassar al-Rubaie, the head of al-Sadr's bloc in parliament, accused U.S. forces of trying to provoke the Sadrists into violence during the expanding campaign to quell Iraq's fighting.

"We condemn strongly the arrest of Sheik Abdul-Hadi al-Darraji. He is moderate and well-known as a media personality and always available in negotiations," al-Rubaie said. "He is a peaceful man and what was mentioned in the American release is lies and justification for the aggression against al-Sadr's movement."

U.S. and Iraqi forces reportedly detained al-Darraji during a raid on a mosque complex before dawn Friday.

The U.S. military, in a statement that did not name al-Darraji, said special Iraqi army forces operating with U.S. advisers had "captured a high-level, illegal armed group leader" in Baghdad's Baladiyat neighborhood, next to the Mahdi Army stronghold of Sadr City. It said two other suspects were detained for further questioning.

Sadiq al-Rikabi, an al-Maliki adviser, told Al-Arabiya television the operation was not coordinated with Iraq's political leaders and was not part of the new security campaign.

Police reported at least 16 Iraqis slain in attacks Saturday. In addition, officials said 29 bodies were found in Baghdad and three in the northern city of Mosul, most of them showing signs of torture - a hallmark of killings by sectarian death squads.

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