Mahdi Karroubi, one of two defeated pro-reform candidates in the disputed June 12 presidential election, said he has received the reports of rapes from former military commanders and other senior officials and he called for an investigation.
"A number of detainees have stated that some female detainees were so severely raped that their genitals were damaged. Others savagely raped young boys so that they suffer from depression and serious physical and mental damage," Karroubi said in a letter posted on his Web site.
He said such crimes, if proven true, would "disgrace" Iran's Islamic ruling system. He did not name any officials who made the rape claims.
Senior police and judiciary officials acknowledged over the weekend that opposition detainees have been abused in prison and called for those responsible to be punished, apparently in an effort to calm public outrage over the mistreatment and death of prisoners.
Both the opposition and some vocal critics within the government's conservative support base have railed against mistreatment and deaths of prisoners, presenting another obstacle to silencing postelection unrest. The opposition claims the election was rigged in favor of President Mahmoud Ahmadinejad and that pro-reform challenger Mir Hossein Mousavi was the true winner.
Iran has confirmed at least 30 people have died in the country's worst unrest since the 1979 Islamic Revolution, though human rights activists say the toll is likely far higher. The country's chief prosecutor said about 200 protesters and opposition figures are still detained.
Karroubi sent the letter to powerful cleric and former President Akbar Hashemi Rafsanjani, who has backed reformists in the election dispute. In it, he asks Rafsanjani to bring the matter to the attention of Supreme Leader Ayatollah Ali Khamenei.
He also called on Rafsanjani to direct one of the clerical bodies he leads to form a committee to investigate the claims. Rafsanjani heads the Assembly of Experts, an elected body of 86 clerics whose official role is to oversee the supreme leader's performance.
Karroubi said he did not send the letter to Ahmadinejad because he considers his presidency illegitimate.
The mistreatment of detainees arrested in the crackdown on protesters has extended far beyond the reformist camp. Influential figures in Iran's clerical hierarchy have also condemned the abuses and the three deaths known to have taken place at the Kahrizak prison, which is at the center of the abuse claims.
Iran's prosecutor general called Sunday for those responsible for mistreatment to be punished and said protesters weren't even meant to be taken to Kahrizak prison, located on the southern outskirts of the capital, Tehran.
The country's police chief Gen. Ismail Ahmadi Moghaddam on Sunday also acknowledged protesters were beaten by their jailers at the same facility but maintained that the deaths in the prison were caused by a virus, not abuse.
The head of the prison has since been arrested along with three guards there and the prison has been closed.
About 100 of the detained opposition figures and protesters are on trial for charges including trying to overthrow the government. U.S. Secretary of State Hillary Clinton called it a "show trial" in a weekend interview and said the U.S. supports the opposition.