Iranian authorities put Iranian opposition leaders Mir Hossein Mousavi and Mehdi Karroubi, along with their wives, under arrest.
Both leaders were under threat after they issued a request to have demonstrations in February. They had emerged as opposition leaders after a failed attempt to unseat Iranian President Mahmoud Ahmadinejad in disputed 2009 elections.
U.S. Sens. Jon Kyl, R-Ariz., and Mark Kirk, R-Ill., said the detention was a sign the ruling regime in Tehran was crumbling.
"Reports that the Iranian regime arrested and imprisoned opposition leaders Karroubi, Mousavi and both men's wives show signs of a weakening dictatorship terrified of its own people," they said in a joint statement. "The regime is wrong to think that this latest despotic act will secure its power."
Iranian lawmakers had called for the execution of both of the opposition leaders. Radio Zamaneh, a Persian broadcaster in the Netherlands, reports that lawmakers Wednesday accused both men of treason. Tehran said there is no reason not to proceed with prosecution.
"We urge the (U.S.) president to immediately and publicly call for the release of the Karroubis and Mousavis and to continue to do so until those presently in control of Iran have released these men and women and all other wrongfully imprisoned political dissidents," Kyl and Kirk added.