Earlier this week, more than 200 Iranian lawmakers called for the arrest and execution of opposition leaders Mir Hossein Mousavi and Mehdi Karroubi, alleging they are collaborating with foreigners and undermining the system.
The New York Times reported Thursday that Mousavi was missing.
His daughter told an opposition Web site the family had no word from Mousavi and his wife since Tuesday and feared they may have been detained, the newspaper said.
Karroubi reported on his Web site that security forces had raided his eldest son's home, the paper said.
Iranian government officials held Mousavi and Karroubi responsible for Tehran protests Monday in which hundreds of anti-government activists called for an end to the regime.
Security forces, deployed in the capital to quell the protests, reportedly opened fire on demonstrators. Two protesters were killed and nine members of Iran's security forces were injured.
Iranian authorities placed Mousavi and Karroubi under house arrest a day before the anti-government protests.
"Mehdi Karroubi and Mir Hossein Mousavi are corrupt on earth and should be tried," one lawmaker told IRNA Tuesday.
The term "corrupt on earth" is a charge used against political dissidents in the past and carries the death penalty in Iran, media reports indicated.
Participants in Friday's planned to "scream out their hatred, wrath and disgust against the savage crimes and evil movement of sedition leaders, their hypocrites and monarchist allies," said a statement released by the Islamic Propogation Coordination Council, which was organizing the pro-regime gatherings.
Senior officers in Iran's Revolutionary Guard wrote a letter to their superiors asking for guarantees they will not be required to open fire on anti-government protesters, Britain's Daily Telegraph reported Friday. The officers said using violence against their own people breaks the tenets of Shiite Islamic law, the newspaper said.
During a visit to Bushehr in advance of a rally in the city in support of his regime, President Mahmoud Ahmadinejad lashed out at U.S. President Barack Obama, Ynetnews.com said.
"Obama wants to rule the world, but he can't even control his own emotions," the Web site quoted the Iranian president as saying. Ahmadinejad said Obama "does not even know how to spell his own name properly," the site said.