"Such massacres uncover the lies of the U.S. administration which pretends to work on granting Iraqis the freedom of expressing their mind and opinion," Sheik Mohammed Hussein Fadlallah said in a statement.
Fadlallah, the mentor of pro-Iranian Lebanese Shiites, urged Arab and Muslim countries "to stand by the side of the oppressed and wronged Iraqi people to help them achieve freedom and justice."
Fadlallah was commenting on Sunday's clashes between U.S. troops and Sadr's Shiite supporters in Baghdad and other parts of Iraq.
Fighting that raged all night in Baghdad's al-Shaala neighborhood killed eight U.S. soldiers and dozens of Iraqis.
The fighting started when coalition troops exchanged gunfire with protesters in the southern city of Najaf, killing four Salvadoran soldiers and at least 20 Iraqis. After that clash, Sadr supporters in Baghdad seized control of five police stations, which were then retaken by U.S. troops.
The protests began after a top aide to Sadr was arrested during the weekend.