Mr Karroubi said if even one account were true, it would be a tragedy
A defeated opposition candidate in Iran's presidential election has called for an investigation into allegations some protesters were raped in prison.
In a letter to former president Akbar Hashemi Rafsanjani, Mehdi Karroubi said senior officials had informed him of the "shameful behaviour" taking place.
Mr Karroubi wrote that both male and female detainees had been raped, with some suffering serious injuries.
He asked Mr Rafsanjani to consult the Supreme Leader about the allegations.
About 200 people arrested during the mass protests sparked by June's disputed election, which saw President Mahmoud Ahmadinejad re-elected by a wide margin, are still being detained.
In the letter addressed to Mr Rafsanjani in his capacity as head of the Assembly of Experts, Mr Karroubi demanded an investigation into allegations that several detainees had been sexually assaulted.
"Some of those arrested [as a result] of the unrest claim that detained girls have been sexually assaulted with... brutality," he wrote.
"The young men in detention were also sexually assaulted in such a way that some are now suffering from depression and other physical and psychological problems, and are incapable of even leaving their homes," he added.
Mr Karroubi said that the people who had told him about the allegations of sexual assault held "sensitive positions".
"Even if one account is true, it would be a tragedy for the Islamic Republic
and it would whitewash the sins of many dictatorships, including that of the deposed Shah," he added.
On Thursday, police confirmed serious rights violations had taken place at the Kahrizak detention centre, where most of those arrested at the protests were sent.
The head of Kahrizak was sacked and jailed on Sunday along with three of his guards, who were found to have beaten detainees.
Supreme Leader Ayatollah Ali Khamenei ordered the closure of the centre in July, because it had failed to "preserve the rights of detainees". Police officials have admitted that some of those held since June might have been tortured.
Earlier on Sunday, a senior commander of Iran's Islamic Revolution Guards Corps said that Mr Karroubi, and the other main defeated opposition presidential candidate, Mir Hossein Mousavi, should be tried for inciting unrest after the election.
"If Mousavi, Karroubi and [former president Mohammad] Khatami are the main suspects behind the soft revolution in Iran, which they are, we expect the judiciary... to go after them, arrest them, put them on trial and punish them," Yadollah Javan told the Irna news agency.
Foreign media, including the BBC, have been restricted in their coverage of Iran since the election protests turned into confrontations with the authorities in which at least 30 people died.