There has been no word from Iranian opposition leaders Mir Hossein Mousavi and Mehdi Karroubi since Feb. 14 when they, along with their wives, were taken by Iranian security forces.
A coordinating council for the opposition movement in a letter to U.N. Secretary-General Ban Ki-moon said the detention "violated the Iranian Constitution and civil law as well as the international provisions of human rights."
The letter claims the opposition leaders are being held without access to the public or their families and their whereabouts are unknown, reports Dutch broadcaster Radio Zamenah.
The opposition council requests the United Nations "use their good offices" to put an end to the "flagrant violation of the rights of these national leaders and free them without delay."
Both opposition leaders were arrested shortly after issuing requests to Tehran to hold public demonstrations. Mousavi was the top challenger to Iranian President Mahmoud Ahmadinejad, whose re-election in 2009 sparked violent protests in Iran.
Iranian lawmakers accused Karroubi and Mousavi of treason and earlier called for their execution.