In a report published by Fars news agency, the panel said its members met Karroubi and found that his allegations were baseless.
"The panel concludes that there is no proof that people who Karroubi alleged of being raped, have been raped," the panel's report said.
"These allegations have been made without any proof, and all the documents given by Karroubi are baseless. These allegations were aimed at distracting public opinion."
The panel report recommended that action be taken against Karroubi and those airing rape allegations.
"This commission proposes … sending its report to the judiciary so it can act with determination against those who are responsible for spreading such allegations which harm the regime," it said.
Karroubi, a reformist who was defeated in the June 12 presidential election, has claimed that several male and female protesters who were jailed during post-election unrest were raped in custody.
Following his claims, Iran's newly appointed chief of judiciary, Sadeq Larijani, set up a panel to probe his allegations.
Karroubi has been the most vocal in airing allegations that protesters had been sexually abused and beaten in police custody. Iranian officials have already dismissed his claims.
Earlier on Saturday, Tehran's military prosecutor said 90 former inmates of an Iranian detention centre closed over allegations of excesses during the post-election crackdown have formally complained of abuse.
Shokrollah Bahrami, quoted by the Mehr news agency, did not give any details on what kind of abuse had been claimed.
The re-election of President Mahmoud Ahmadinejad triggered the worst crisis in the 30-year history of the Islamic republic as massive public demonstrations against his victory rocked Tehran.
Iranian officials say 36 people were killed during the unrest, but opposition groups claim that 72 died.
