Iran's security prosecutor questioned on Monday two daughters of leading opposition figure Mir Hossein Mousavi, who has been under house arrest for nearly two years, the ISNA news agency reported.
Zahra and Narges Mousavi were summoned to "the security prosecutor ... to give some explanations," a spokesman for the Tehran prosecutor said, quoted by ISNA, without giving details on the interrogation.
The two returned home later the same day, ISNA quoted Mohammad Saleh Noqrekar, a relative of Mousavi, as saying.
Noqrekar said the sisters had "spent a few hours in the prosecutor's office at Evin," Tehran's notorious prison housing political prisoners as well as detained journalists and activists.
Earlier Kaleme.com, a website close to Mousavi, said the sisters had been arrested by security forces who raided their house. However, the report was denied by the Tehran prosecutor, ISNA reported.
Mousavi was a leading figure behind 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 charged the election was rigged and supported a string of protests against Ahmadinejad.
The pair -- branded by authorities as "leaders of the sedition" which sought to overthrow the government after the election -- have both been kept under house arrest since February 2011.
The UN's human rights monitor for Iran on Monday expressed concern about the reported detentions, and called for the release of Mousavi and his family.
"I urge the Iranian government to immediately and unconditionally release the two opposition leaders and their family members, and to end all restrictions on their movement and legally protected activities," said Ahmed Shaheed.
The latest developments come as Iran prepares to hold June presidential polls to elect a successor for Ahmadinejad, who cannot run for a third consecutive term under Iranian law.