The cleric, who continues to question the legitimacy of President Mahmoud Ahmadinejad's re-election, said it was still difficult for him to understand how the hardliner won the poll last year given his government's track record.
"Unfortunately, the (Islamic) republic has been plagued with despotism and elections have become meaningless. It has become only a term," Karroubi told a group of visitors from central province of Isfahan, according to his website Samannews.
"How can one believe that a president with so many objections against him such as inflation, unemployment… gets more votes than he got in his first election?"
Ahmadinejad has been accused of stoking inflation with a populist economic policy that has involved pumping large sums of money into the economy.
Karroubi's remarks came two days after hardliners reportedly gathered outside his Tehran home, calling for him to be put to death.
His wife, Fatemeh, charged that a group of "thugs" paid by "corrupt" government officials had vandalised the apartment block where the family lives.
Iran's Fars news agency described the small but vocal crowd which gathered outside the flats as "students and families of martyrs" of the Iran-Iraq war.
Pictures carried by the pro-government Borna news agency showed the building defaced with red colouring while slogans pronouncing "Death to Karroubi" were scribbled on the walls.
Karroubi and former prime minister Mir Hossein Mousavi have led a protest movement against Ahmadinejad since his June re-election, which they reject as massively rigged.
Karroubi was attacked by hardliners during Iran's commemoration of the Islamic revolution of 1979 on February 11 and his car was shot at in January in the city of Qazvin, west of Tehran.
The outspoken cleric, who with Mousavi stood against Ahmadinejad in the June vote, has infuriated hardliners by charging some post-election detainees had been raped in jail. Iranian authorities vehemently deny the allegations.
