Former Iranian presidential candidate Mehdi Karroubi, under house arrest since 2011, appealed to President Hassan Rouhani on Sunday for a court hearing where he could show two elections he lost were rigged.
Karroubi, a former speaker of parliament, said Iran's powerful Revolutionary Guard, Basij militia and Iranian Intelligence Ministry falsified the results of the 2005 and 2009 polls, which resulted in a Mahmoud Ahmedinejad victory.
Karroubi also said Iranian authorities used "thugs" to attack political critics, embassies and cultural centres.
Critics of authorities in Iran, particularly from a politician like Karroubi, are rare.
"The only path is reform and returning to the direction of the law," Karroubi wrote in the letter.
He also said he is prepared to accept whatever sentence the court issues.
Since 2011, Karroubi and fellow reformist presidential contender Mir Hussein Moussavi, have been under house arrest.
President Rouhani suggested during his 2013 election campaign that he might free the two political prisoners, but has not taken up the issue due to the fact that it could lead to a showdown with hardliners opposed to him.
Karroubi and Moussavi have been accused of igniting an uprising after the 2009 polls. Neither of the two figureheads of the country's "green movement" have been formally charged or had their day in court.