Three UN human rights watchdogs on Monday expressed concern about Iran's reported arrest of family members of detained opposition figure Mir Hossein Mousavi, who they said should also be set free.
In a joint statement, the monitors said Iran should release Mousavi's daughters, detained Monday, plus Mousavi and his wife, and fellow opposition leader Mehdi Karroubi.
"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, UN human rights monitor for Iran.
Karroubi and Mousavi, along with Mousavi's wife Zahra Rahnavard, have been under house arrest since February 2011.
The Iranian news agency ISNA reported Monday that the public prosecutor had summoned two of Mousavi's daughters for questioning but denied they had been arrested.
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 Karroubi charged the election was rigged and guided a string of protests against Ahmadinejad.
The pair were branded by authorities as "leaders of the sedition" which sought to overthrow the government after the election.
"None of these people have been charged with a crime since their arrest, or have been brought before a judge to contest the legality of their detention," said El Hadji Malick Sow, the UN's monitor on arbitrary detention.
"This is in violation of Iran’s international obligations and national laws, which provide that no one shall be subjected to arbitrary arrest or detention."
The latest developments come as Iran prepares to hold a June election to elect a successor for Ahmadinejad, who cannot run for a third consecutive term under Iranian law.
Maina Kiai, the UN monitor on the right to freedom of assembly and association, said Iran would be under scrutiny.
"Such rights are essential components of democracy and are all the more relevant in the context of Iran’s upcoming presidential elections," he said.