Mousavi in a sweeping statement on his Kaleme.com Web site calls for a political reforms in Tehran, the BBC reports.
In his letter, the former prime minister calls for an "end to the involvement of police and military forces in politics, the independence of the judiciary, and prosecution of those in plainclothes (Basij militia)."
Iranian observers have said recently that the Iranian military has grown increasingly influential in Iranian politics.
Mousavi's statement came as Iran moves past the first anniversary of deadly clashes that coincided with the contested victory of Mahmoud Ahmadinejad in June 2009 elections.
Scores of police and plainclothes militiamen poured into the streets of downtown Tehran in June 2009 in response to mass demonstrations. More than 100 activists were arrested and several executed after trials in which they were convicted a various demonstration-related charges.
Mousavi and fellow opposition leader Mehdi Karroubi last week called off anniversary demonstrations out of fear of a government backlash.
Mousavi, in his memo, calls on authorities to hold a trial for election violations.
"A fair trial of those who committed the election fraud, tortured and killed protesters must be held," the BBC quoted the letter as reading.