Karroubi, a former speaker of the Iranian Parliament, challenged Ahmadinejad in contested June presidential elections. He finished fourth, joining opposition leaders in claiming the incumbent stole the elections.
His son, Hosein Karroubi, told the Fars News Agency, however, that his father recognizes Ahmadinejad as the "head of the regime's government" because of the consent given by Supreme Leader Ali Khamenei.
He stressed, however, that his father thinks the June ballots were "rigged," citing "interference" in the counting of the votes.
Despite a requirement for a hand count of all votes, Ahmadinejad declared victory in the election contest within hours of the closing of the polls. His main challenger, opposition leader and former Prime Minister Mir Hossein Mousavi, led charges claiming the election results were manufactured.
Karroubi said that despite his recognition of Ahmadinejad as the president of Iran, he stood by the basic tenets of the opposition movement.
"We are standing and will not back off from defending people's demands which are nothing but the original demands and reforms of the Islamic Revolution," he said.