Security forces on Monday arrested two daughters of opposition figure Mir Hossein Mousavi who along with his wife is under house arrest, an opposition website reported.
"Security forces raided the home of Zahra and Narges, and arrested them in the morning," Kaleme.com, which is close to Mousavi, said without citing a source.
The judiciary is yet to comment on the report.
Mousavi was a leading figure guiding opposition protests against the result of the 2009 presidential election, which gave President Mahmoud Ahmadinejad a second four-year term.
Mousavi and fellow reformist candidate Mehdi Karroubi said the election was rigged and have guided a string of protests against Ahmadinejad.
They pair -- branded by authorities as "leaders of the sedition" which sought to overthrow the Islamic regime after the 2009 election - are both under house arrest since February 2011.
The report of the arrests comes as Iran prepares to hold in June its next presidential election to find a successor for Ahmadinejad, who by law cannot run for a third consecutive term.