A group of Iranian reformists is demanding an investigation into Supreme Leader Ayatollah Ali Khamenei, questioning whether he is fit to lead.
A letter, issued Friday, asks one of Iran's highest clerical bodies to examine whether the supreme leader has become incapable of fulfilling his "constitutional duties."
Under Iran's constitution, the Assembly of Experts has the power to appoint the supreme leader or remove him from power, although until now challenging the supreme leader's authority has been a political taboo.
Discontent with Ayatollah Khamenei has been growing since June's disputed presidential elections. At the same time, there are increasing allegations that protesters arrested following the election were raped and tortured.
Defeated reformist candidate Mahdi Karroubi this week accused prison authorities of raping detainees and torturing them to death.
However, Senior Cleric Ahmad Khatami denounced the allegations during Friday prayers and called for Karroubi to stand trial for the remarks.
The human rights group Amnesty International is calling for an independent investigation into the allegations of prisoner abuse. The group said Saturday denials of wrongdoing by Iran's leaders do not have credibility since details of their own investigations have not been released.
More than 100 detainees have been involved in mass trials during the past two weeks. Iranian Nobel Peace Prize laureate Shirin Ebadi calls them "show trials" that must be stopped.
Iranian President Mahmoud Ahmadinejad was sworn-in for a second term earlier this month, but opposition leaders, including Karroubi and Mir Hossein Moussavi say the results were rigged.
Meanwhile, Ayatollah Khamenei Saturday appointed the brother of Parliament Speaker Ali Larijani to head the country's judiciary.
Ayatollah Sadeq Larijani will serve a five-year term.
Some information for this report was provided by AFP, AP and Reuters.