Supporters of Mir Hossein Musavi and Mehdi Karrubi say they and their wives were taken from their homes in recent days and transferred to a jail in the capital, Tehran.
Iranian authorities have denied this, saying the two couples are still in their homes.
Western officials have condemned the reported detention of Musavi and Karrubi and called for their release.
Reza Moini, head of the Iranian section of Reporters Without Borders, told Radio Farda that the secretive arrest of political prisoners is against international law.
"We have questioned the Iranian authorities regarding this matter," he said, "and have demanded the immediate disclosure of the arrest situation and location of these figures as per international law and the Iranian Constitution suggesting that the charge, along with the reason [for the charge], should be announced upon every arrest."
Moini said his group has kept track of the many threats it says have been made against Iranian intellectuals both inside and outside Iran in recent years.
"We have asked for international assistance in this matter, and have demanded the safety of Iranian journalists both inside and outside the country," he added.