In a statement posted on opposition Web sites - Mousavi''s own Kaleme site and Jaras - Friday, the reformer said Iran is in the midst of a "serious crisis."
Mousavi warned jailing or killing opposition leaders will not calm the situation. He also called on the government to adopt a five-point plan, which includes freeing political prisoners and establishing new, transparent election laws.
The statement follows some of the worst violence to hit the country since June, when President Mahmoud Ahmadinejad won a disputed election.
On Thursday, Iran increased security in the capital, while the Intelligence Ministry warned it would deal with those who incite riots or violence in the country.
Protests last Sunday turned bloody when Iranian security forces and opposition activists clashed. At least eight people were killed in the violence, which coincided with Shi''ite Islam''s solemn Ashura holiday.
Iranian opposition Web sites reported sporadic skirmishes Thursday, but those reports could not be confirmed. Foreign media have been banned from directly covering opposition activities in the streets.
On Wednesday, large pro-government crowds shouted death threats to opposition leaders they blame for unrest in the Islamic republic.
Conservative clerics have accused opposition supporters of being enemies of god or "mohareb," an offense that carries a death sentence.