"Dictatorship in the name of religion is the worst kind. The most evident manifestation of a continued tyrannical attitude is the abuse of parliament and judiciary. We have completely lost hope in the judiciary," he said.
Mousavi added that he no longer believed, as he once did, "that the revolution had removed all those structures which could lead to totalitarianism and dictatorship."
"Stifling the media, filling the prisons and brutally killing people who peacefully demand their rights in the streets indicate the roots of tyranny and dictatorship remain from the monarchist era ... I don't believe that the revolution achieved its goals," he said.
Mousavi, who has spearheaded the anti-government protests since the disputed re-election of President Mahmoud Ahmadinejad last June, made the remarks as Iran marks the 31st anniversary of the Islamic revolution.
Celebrations marking the 1979 return from exile of hardline cleric Ayatollah Ruhollah Khomeini began on Monday and will climax on February 11, the anniversary of the fall of the shah, who had ruled Iran for nearly four decades.
The violent protests against Ahmadinejad have triggered one of the worst crises since the foundation of the Islamic republic and divided the nation's clergy.
Dozens of people have been killed, hundreds wounded and scores put on trial by the authorities, accused of plotting to overthrow the government.
The Fars news agency reported that Iran's deputy judiciary chief Ebrahim Raisi said Tuesday that nine people arrested during anti-government protests would "soon" be executed for what he said was their bid to topple the Islamic regime.
Iran hanged two people on Thursday on similar charges.
Their executions were condemned internationally and branded by opposition leaders as an effort to scare protesters and keep them off the streets during the traditional February 11 rally marking the anniversary of the revolution.
Mousavi and Mehdi Karroubi, Iran's other main opposition leader, have called for a large turnout by their supporters at the rally, prompting the security forces to warn of a massive crackdown should the occasion be used to stage anti-government protests.
Eight people were killed, among them Mousavi's nephew, and about 1,000 arrested in the last anti-government demonstration on December 27.
The two opposition leaders maintain that Ahmadinejad's re-election was "fraudulent" and Karroubi has even alleged that several protesters detained in the immediate aftermath of the June election were raped in jails.
But on January 25 Karroubi's son Hossein told AFP that his father now recognised Ahmadinejad as "the head of government" as his re-election was endorsed by supreme leader Ayatollah Ali Khamenei.
In an interview with MSNBC television, the US vice president sharply criticised the Iranian regime's handling of the protests but defended the administration's carrot and stick approach.
"I think we've moved in the right direction ... We're going to end up much better off than we would have had we tried to go in there and physically tried to change the regime," Biden said.
"But, look, when they acted as they did, when the first protests broke out and people were brutalized, they lost their moral credibility and their own country and around the region. And I think they're sowing the seeds for their own destruction."