Opposition Web site Jaras reports that Karroubi and his wife were taken from their home Friday and nothing is known as to their whereabouts.
"An eyewitness had gone to the alley were Mehdi Karroubi's home is located and noticed that not only were the lights completely off and there was no sign of anyone's presence, but also that there was no longer any sign of security vehicles or the plainclothes forces who used to patrol the area," Dutch broadcaster Radio Zameneh reported, citing the Persian-language account.
Iran is cracking down on the opposition movement as political uprisings spill across much of the Middle East and North Africa.
Tehran greeted the waves of unrest that followed the contested 2009 re-election of Iranian President Mahmoud Ahmadinejad with a show of force.
Masked security forces took the place of the guards normally posted outside the home of Iranian opposition leader Mir Hossein Mousavi in mid-February.
Mousavi was put under house arrest Feb. 14 as opposition leaders in Iran petitioned the government to stage demonstrations.
Lawmakers have called for the execution of Mousavi and Karroubi. Both men launched bids to unseat Ahmadinejad in 2009 and became leaders of the opposition Green Movement in the wake of the unrest.