Libyan Prime Minister Abdurrahim El-Keib (R) talks with Foreign Minister Ashour bin Khayyal (L)
Tunisian President Moncef Marzouki has said that his country's extradition of Libya's former Prime Minister al-Baghdadi al-Mahmoudi was "illegal".
Mahmoudi, 70, had fled to Tunisia during the uprising against Col Muammar Gaddafi last year, reports BBC.
Marzouki's office said in a statement that the decision to extradite Mahmoudi was taken without consultation and without his agreement.
Mahmoudi is the first senior Libyan official to be returned for trial.
Marzouki had promised earlier this year to hand over Mahmoudi if he were guaranteed a fair trial in Libya, but in May he said he was in principle opposed to the move.
Human rights groups had asked Tunisia not to extradite him, saying he could be subject to human rights violations in Libya.
A Tunisian government statement said the extradition did not require the president's signature.
It said the decision to extradite had been made following a report by a Tunisian delegation to Tripoli, which found that the conditions for a fair judicial process had been met.
However, the statement from Marzouki's office insisted that the decision affected foreign policy, which is "within the purview of the president of the republic".
The extradition "threatens the image of Tunisia in the world, making it appear like a state which is not committed to... respecting the elements of a fair trial," the statement went on.
Mahmoudi was transferred by helicopter on Sunday.
He had served as prime minister from March 2006 but fled Libya when Col Gaddafi's forces lost control of Tripoli and was arrested in Tunisia in September for illegal entry.
A sentence of six months' imprisonment for that offence was overturned on appeal, but Mahmoudi had remained in Tunisian custody following the extradition request from Libya.
Mahmoudi's lawyers have expressed fears for his life, saying he now has sole knowledge of many of the state secrets from Gaddafi's time in power.
Libya's new government has pledged to treat all detainees fairly.