Defeated candidate Karoubi, who finished fourth in the June 12 vote, angered hardliners in August by saying some detained protesters had been raped.
A top judicial investigative committee rejected Karoubi's allegation on Saturday and reportedly called for libel charges to be considered against anyone making such claims.
"I feel obliged to resist and not to give up my fight (for those who have been mistreated in jail)," said Karoubi in a statement published on his Etemademelli website on Tuesday.
"In my worst dreams I could never imagine a day will come when the Islamic Republic reacts to the people's peaceful rallies … with batons and bullets."
Karoubi and another defeated candidate Mirhossein Mousavi say the poll was rigged to secure President Mahmoud Ahmadinejad's re-election. Officials reject the charge.
The election and its turbulent aftermath plunged Iran into its deepest internal crisis in the past three decades. Rights groups say thousands of people, including senior reformers, were arrested after the poll, though most have been freed.
Hardliners have called for Karoubi to be arrested or charged if he fails to back up his accusations.
"The judicial committee calls for firm confrontation with me … I welcome this idea as an opportunity to reveal more details about the issue," Karoubi said.
Any legal action against Karoubi may lead to fresh protests because the mid-ranking cleric enjoys widespread support.
Supreme Leader Ayatollah Ali Khamenei in July ordered the closure of Kahrizak detention centre south of Tehran, following reports of abuse of people who were detained in street unrest that followed the presidential poll.
At least three people died in custody there and last month the authorities said the head of Kahrizak had been jailed, as well as three policemen who beat detainees.
The opposition says more than 70 people were killed in post-election unrest. Officials estimate the death toll at up to 36 people including members of the Islamic Basij militia.
The authorities have portrayed the opposition protests as a foreign-backed bid to undermine the Islamic government system.
Iran last month began mass trials of more than 100 people, including a former vice president and other former top officials, accused of fomenting huge protests after the vote. (Writing by Parisa Hafezi; Editing by Louise Ireland)
