"We are not against the Islamic system and its laws [but] against the deviations and lies, and we want to reform them; a reformation that returns us to the pure principles of the Islamic revolution," Mousavi said in a statement late on Saturday.
He said the record 85 percent voter turnout for the presidential race was because of new hope to deal with "widespread social dissatisfaction" that "can target the bedrock of the revolution and the Islamic system."
"If the high volume of cheating and vote manipulation" were to stand, Mousavi said, "the republicanism of the regime will be slaughtered and the idea of incompatibility of Islam and [democracy] would be practically proven."
While Mousavi may be limiting his ambition to a re-run of the election – which Khamenei all but ruled out in an uncompromising sermon during Friday prayers – damage to Iran's ruling system has been done. The political crisis has exposed many fissures within the establishment and across a divided society.
"These days and nights a turning point is being forged in the history of our nation," said Mousavi.
'Crisis of legitimacy'
"The Supreme Leader's speech on Friday deepened that crisis of legitimacy, and deepened that crisis of unity over the whole Islamic establishment," says Massoumeh Torfeh of the School of Oriental and African Studies in London. "It was a dangerous speech because he's now played all his final cards. He cannot go back now.
"The opposition has a clear choice: Either it has to be quiet enough for a while, and work things out and come back with a plan and a strategy. Or it will be heavily confronted by force, as it was [Saturday]" says Ms. Torfeh. "I think they will keep it intact.... But [with] a discredited Supreme Leader, and a totally unaccepted president, it is going to create serious crisis for the establishment."