The meeting with relatives of Emadeddin Baghi, the award-winning founder of the now-banned Tehran-based Society to Defend Prisoners' Rights, took place on January 6.
Baghi, 49, was sentenced last year to seven years in prison, including six on charges of "propaganda against the system" over an interview he recorded with the late Grand Ayatollah Hossein Ali Montazeri that was broadcast by BBC Persian after the cleric's death in 2009.
Ali Maghami, Baghi's son-in-law, was also arrested by security forces on December 27 on unclear charges.
Mojtaba Vahedi, a U.S.-based analyst and former editor in chief of the reformist Iranian newspaper "Aftabe Yazd," told Radio Farda it was likely Baghi's activities on behalf of prisoners' rights that prompted Karrubi's interest in his case.
"Karrubi, despite being aware of the sensitivity of the issue, has always had this habit of meeting and sympathizing with the families of political prisoners," Vahedi said.
Baghi has spent 4 1/2 years in prison in recent years.
He was awarded the Geneva-based Martin Ennals Award for Human Rights Defenders in 2009.