"This morning at Tunis-Carthage airport, I was prevented from leaving Tunisian territory without having been notified beforehand of any judicial decision," Fadhel Abdelkafi, head of the centre-right Afek Tounes party, wrote on Twitter.
A statement from the party called the move against the former development minister "a violation of basic and constitutional freedoms".
"This illegal measure is further proof of the growth of political domination and the accelerated deviation of President Kais Saied's regime towards dictatorship", the statement read.
An Interior Ministry official told Reuters a Tunis court had issued a judicial decision to prevent Abdel Kefi from travelling, adding that the ministry was not empowered to take such a step independently.
Abdelkafi told Reuters he had not been made aware of any judicial decision against him. "Is it reasonable for a decision to be issued without my knowledge?" he said, calling the move a "violation of a basic right".
The Afek Tounes party said Abdelkafi's travel ban was "repressive", showing that "state institutions are being instrumentalized to suppress the opposition and target political figures."
Afek Tounes has been a principal opponent of Saied, whose dramatic power grab in July last year prompted accusations he was seeking to restore an autocracy in Tunisia, a decade after a revolution that sparked the Arab Spring uprisings.
The move comes a day after police started investigating a domestic journalist over an article critical of the prime minister and in the run-up to legislative elections that Saied's opponents have decried as undemocratic.
This report was prepared with data from Reuters and Agence France-Presse.