In an interview with an opposition website, Mousavi compared the Ayatollah regime ruling Iran to other tyrannical regimes in history, including the Nazi regime.
They are resorting to methods (against the opposition) used in totalitarian regimes like Stalin in the Soviet era or (former dictator Nicolae) Ceausescu in Romania," Mousavi said in a statement posted on his website.
He added that the propaganda statements of Joseph Goebbels in Nazi Germany paled in significance to the lying done by Iran's rulers.
"They've surpassed Goebbels in telling lies. Leveling accusations and telling lies is part of their ossified faith," he said.
Tehran's chief prosecutor, Abbas Jafari Dowlatabadi, said last month that it was only a matter of time before opposition leaders are put on trial for the unrest following the disputed 2009 presidential election. Iranian officials have slammed opposition figures and characterized them as "traitors."
So far, however, authorities have stopped short of trying to jail the reform movement's top leaders, possibly out of concern it could spark a new wave of protests and fuel the opposition.
Iran's opposition leaders had been highly critical of the regime in recent months, last month warning that a "dark future" awaits the local economy because the government didn't listen to economists when it slashed energy and food subsidies in a country already struggling under biting UN sanctions.
"Enforcing this plan, while Iran is facing tough international sanctions and its economy is in recession with an unemployment rate of over 30% and wild inflation, is a burden on medium and low-income families," Mousavi and fellow opposition leader Mehdi Karroubi said at the time in a statement posted on website kaleme.com.
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