"The Green movement will not abandon its peaceful fight," he said in comments published on his Kalameh Web site, "until people's rights are preserved. Peaceful protests are Iranian's right."
In an explicit challenge to the government, Mousavi said that today Iran is worse off than it was during the reign of the Shah.
"Parliament is too weak to perform its tasks, some [members] are worse than what we had at the Shah's time," he said. "Authoritarians will roll us back to the darker days before the Revolution."
"It is the legitimacy of key players including the Supreme Leader which is being challenged increasingly openly," said Dr. Claire Spencer, an expert on Iran with the British think tank Chatham House. "This is a way of saying the authenticity has been undermined by the recent government [and asking] what is the legitimacy of the Islamic Republic?"
His comments follow the executions last week of two men, Arash Rahmanipour, 19, and Mohammad-Reza Ali-Zamani, 37. They were arrested in the riots that followed the presidential elections in June. In a final tally many claim was rigged, President Mahmoud Ahmadinejad won a second term in office. Rahmanipour and Ali-Zamani were sentenced to death on charges of "waging war against God." Iran today announced it would hang nine more men taken into custody on the same charges.
"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," Mousavi said on his Web site. "I don't believe the revolution achieved its goals."
His comments come as tensions between the government and thousands of Iranians calling for regime change persist. Both Mousavi and his Mehdi Karroubi, his reformist ally, condemn the hangings and accuse the government of trying to intimidate the opposition.
On February 11 the country will mark the 31st anniversary of the founding of the Islamic Republic. Mousavi and Karroubi are encouraging supporters to attend peaceful rallies that day calling for an end to dictatorship.
"In the past Mr. Mousavi has been cautious about encouraging people because the death toll has been a clear risk for anyone going out, even peaceful demonstrators," said Dr. Spencer.
In December, the streets were filled with protestors during the Shia ritual of Ashura. Eight Iranians were killed, including Mousavi's nephew Seyed Ali Mousavi. Over 1,000 people were arrested.
Some observers say Mousavi's comments suggest he is challenging the government to arrest him. He goes so far as to call for changes to the constitution. Last month, Mousavi said he was not afraid to die for the cause of reform.
Afshin Abtahi contributed to this report.