Iran has arrested opposition leaders Mirhossein Mousavi and Mehdi Karoubi, the opposition website Kaleme said on Monday.
Sources say that they have been arrested and transferred to Heshmatiyeh jail in Tehran, Mousavi's website Kaleme reported.
Judiciary officials were not immediately available for comment.
The International Campaign for Human Rights in Iran, an advocacy group which has staff in the United States and Germany, said on Sunday the two leaders had been moved secretly from their homes where they had been under virtual house arrest for calling on supporters to protest against the government.
Mousavi and Karoubi had been forced to stay in their homes in the capital Tehran for more than two weeks. Mousavi's daughters said on the Kaleme website that they had been prevented from approaching the house since February 14.