Kaleme, a Web site used by the Iranian opposition, reported in February that Karroubi and his counterpart Mir Hossein Mousavi were taken to the Heshmatiyeh prison in Tehran.
Hadi Ghaemi, director of the International Campaign for Human Rights in Iran, was quoted as saying by CNN that the opposition leader is being brainwashed while in custody.
"We are extremely concerned for the health and well-being of Karroubi, who is 74 years old, and no one has heard from him for six weeks, not his wife, any family or associates," he said.
Both opposition leaders were arrested shortly after issuing requests to Tehran to hold public demonstrations early this year. 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.
Karroubi's wife said she hasn't been in contact with the opposition leader since July 16.