Najaf, Iraq ? U.S. forces raided the headquarters of radical Shiite Muslim cleric Muqtada al-Sadr on Tuesday in the heart of the holy city of Najaf and arrested his top advisers in the strongest blow yet to al-Sadr’s nationwide insurgency.

The predawn raid drew an angry rebuke from the country’s top Shiite cleric, whose support is vital to maintaining calm among the country’s Shiite majority.

“We’ve informed the Iraqi government of our rejection and our condemnation of American forces for entering the holy city of Najaf and approaching the holy shrine,” said a statement released by the staff of Grand Ayatollah Ali al-Husseini al-Sistani in Najaf. “We believe there was no justification for such a military measure and hold the interim Iraqi government responsible for what happened.”

Al-Sadr remained in hiding Tuesday, as did his remaining advisers. He has kept a low profile since his gunmen vacated the city last month under an agreement brokered by Sistani.

Residents said that dozens of troops supported by helicopters stormed the office in which al-Sadr’s advisers were holed up, less than 200 feet from the Grand Imam Ali Shrine. Arrested were Sheik Ahmed al-Sheybani, the most visible among al-Sadr’s inner circle, and his main Friday prayers leader and another key adviser, Hossam al-Husseini.

The clerics, along with several guards, were taken away to an undisclosed location. Witnesses said that Iraqi police later took about 40 Kalashnikov assault rifles from the office.

The raid was the third in five days on al-Sadr’s deputies and offices. In response to an arrest of a spokesman Saturday, an Islamist group seized 18 Iraqi national guardsmen and threatened to execute them. Al-Sadr intervened, and the guardsmen were released Monday.

Many Iraqis believe the raids on al-Sadr’s associates are part of the U.S. campaign to prevent guerrillas from scuttling Iraqi elections early next year.