Mehdi Karroubi made the charge Thursday, days after he said a number of male and female prisoners have been raped and abused, the BBC reported. He called for the creation of an independent committee to review the evidence in a "calm atmosphere."
"Some young people are beaten to death just for chanting slogans in (post-election) protests," Karroubi claimed on his Web site. Iranian officials have denied rapes occurred in Iran's prisons, but admitted that some prisoners were abused.
While his claims have been supported by human rights groups, the speaker of Iran's parliament, Ali Larijani, dismissed the charges as "totally baseless," the British broadcaster said.
"Based on parliament's investigations, detainees have not been raped or sexually abused in Iran's Kahrizak and Evin prisons," Larijani said.
Iranian leaders said Tuesday more than 4,000 people were detained during the protests that took place after the June 12 election, which opposition leaders said was rife with ballot fraud. Authorities said 3,700 of the detainees were released within a few days of arrest.
Iran is in the midst of trials of more than 100 prisoners over their alleged involvement in the protests. On trial are leading opposition leaders, activists, journalists, lawyers, foreign embassy workers and two people with foreign nationalities, the BBC said. The trials have been criticized by Western countries, opposition groups and human rights organizations.