"Unfortunately some of the people who took part in the protests were tortured," Karroubi said on the website of his political party, Etemad Melli.
"I heard they (security forces) stripped people in Kahrizak and made them crawl like animals with prison guards riding on their backs," the reformist cleric said of the detention centre which was closed last month.
He said it was a "shame" for the Islamic republic to indulge in such tactics as "some of those arrested were forced to be naked and piled upon each other" in the prison cells.
"I also heard that while they were being tortured, the protesters were forced to curse their mothers," he added.
In a separate claim in his newspaper Etemad Melli on Saturday, Karroubi also said that "some youngsters who were chanting slogans were beaten in such a way that they lost their lives."
Earlier this week Karroubi alleged that several male and female detainees were also "savagely raped" in prisons. However, these charges were dismissed by parliament speaker Ali Larijani.
Karroubi's allegations have added to an already tense political situation in Iran.
Mahmoud Ahmadinejad's disputed re-election as president has shaken the pillars of the Islamic regime after his defeated challengers — including Karroubi — charged that his victory was a result of mass rigging of votes.
