Nephew of Iranian opposition leader Mir Hossein Mousavi's Rza said his brother's dead body was taken from the Ibn Sina Hospital, the Parliamentary Reformist's Web site parlemannews.ir reported.
"We are searching for the body," he said. "But no one is taking responsibility."
"We will not hold any funeral organizations until the body found," Mousavi added.
Opposition supporters and police clashed on Ashura Day in Tehran. According to international news agencies, the riots resulted in the deaths of 15 people. Police arrested 300 people.
According to the Fars news agency, nephew of Iranian opposition leader Mir-Hossein Mousavi Ali Habibi was murdered during the clashes.