Karroubi, a former parliamentary leader and presidential candidate, said the political scene in Tehran is in the midst of a crisis not seen since the Islamic Revolution in 1979.
"The current conditions are very tense and very difficult," he said in an interview with al-Arabiya. "It has never been like this during the last 30 years of the revolution."
Karroubi along with former Prime Minister Mir Hossein Mousavi emerged as opposition leaders in Iran following last year's contested presidential elections.
Supporters of Mousavi and Karroubi took to the streets after election officials declared Iranian President Mahmoud Ahmadinejad the victor hours after the poll closed in June 2009.
Iran's human rights record came under fire after authorities in Tehran battled opposition supporters in the streets and rounded up scores of political activists, sentencing many to death.
Karroubi told al-Arabiya the regime in Tehran depends on chaos to survive.
"The authority considers that the continuation of its rule depends on creating crises," he said.
The opposition leader said that he and his fellow candidates didn't expect fraud or political turmoil to follow their bid for the presidency.