The response to a speech Friday by Ayatollah Ali Khameini said the government is taking the country "to the slaughterhouse," CNN reported. CNN said it could not confirm that the statement, posted in Farsi, was actually from Mousavi.
Khameini said President Mahmoud Ahmadinejad had won last week's presidential election by such a huge margin over Mousavi that even some electoral fraud would not make a difference to the results.
"If the huge volume of vote-rigging -- which has set aflame people's trust in Iran's ruling system -- is going to be used as proof for the lack of rigging, then the system is going to the slaughterhouse," the post said. "It will also prove the incompatibility of Islam and democracy."
Earlier Saturday, a post on Mousavi's page said he is ready for "martyrdom."
Critics of last week's elections say Ahmadinejad's lopsided victory does not match polls shortly before the vote that showed Mousavi in the lead. Ahmadinejad was also said to have won in every part of the country, including Mousavi's home region.
The government press declared Ahmadinejad the winner before the polls closed.
Since the election, thousands of people have taken to the streets to protest the results and there were reports Saturday that at least 19 people -- and perhaps as many as 150 --had been killed in protest-related violence.