Despite a crackdown that largely quelled the protests following President Mahmoud Ahmadinejad's disputed re-election in June, Mousavi has remained defiant, calling the government "a cult that has no respect for Iran's national interests."
Supreme Leader Ayatollah Ali Khamenei, the most powerful figure in Iran's complex Islamic system, criticized opposition leaders on Thursday for refusing "to bow before the law," saying they had lost "the privilege of being part of the system."
Hardliners have accused reformist opposition leaders of inciting unrest and called them enemies of God, a crime punishable by death under Iran's Islamic law.
But Mousavi said the security crackdown would backfire.
"Millions of Iranians face censorship, obstruction of their freedoms and repressive measures … such measures will distance us from adopting logical solutions," Mousavi said on his Kalemeh website.
"If the issue is not resolved logically, the drop in the system's legitimacy will be accelerated."
Mousavi said the government was "incompetent" to rule.
"The nation that faces an adventurous, war-mongering foreign policy and destructive economic policy … wants changes," he said.
Iran faces growing Western calls for targeted sanctions against it after Ahmadinejad ordered production of higher-grade uranium, stirring fears that Tehran aims to make nuclear bombs, not just fuel for civilian use as it says is the case.
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The June election and its aftermath have plunged the Islamic Republic into the most serious internal crisis in its 30-year history and created a rift within the ruling establishment.
Mousavi said the country was in "crisis."
"Can the government see a solution in intimidating the nation?" he asked. "Guaranteeing free, competitive and healthy votes, are the key points in resolving the issue."
Mousavi said the reform movement was still alive despite pressure from the hardline rulers.
Opposition supporters have defied government warnings against staging illegal rallies and have sought to hijack official demonstrations to hold anti-government protests.
"We will use all legal ways, including legitimate and peaceful street rallies," Mousavi said. "We are steadfast in our demands."
Since June, thousands of protesters and reformers have been arrested. Most have since been freed, though dozens, including former senior officials, have been jailed for up to 16 years. In January, Iran hanged two people sentenced to death in post-vote trials. At least nine others are appealing death sentences.
But authorities have so far not dared to directly touch Mousavi, despite his continued defiance.
Mousavi challenged Khamenei, calling a February 11 rally to mark anniversary of the 1979 Islamic revolution "engineered."
Khamenei thanked the Iranian people for turning out in "tens of millions," calling the rally "a slap on the face of enemies."
"There is no pride in holding such an engineered rally. It is exactly like the despotic methods used by shah before the revolution," Mousavi said.
"You cannot … bus people to the rally and then be happy and say everything is under control and over."
(Writing by Parisa Hafezi; Editing by Jon Hemming)
