Unconfirmed reports from Iran say senior opposition figures Mehdi Karoubi and Mir Hossein Mousavi have been moved to a secret location near Tehran.
Supporters of the two leaders say they were recently placed under house arrest following renewed anti-government protests in Iran. Tehran has neither confirmed or denied the accusations.
Former reformist president Mohammad Khatami has urged authorities to release them. The opposition has called for fresh demonstrations every Tuesday until they are freed.