In a message on social media, Libyan official el-Mechri deplored the detention in Tunisia "of the speaker of the parliament elected by the people, the thinker Rached Ghannouchi, because of an opinion he expressed".
"This absurd development bears witness to the return of dictatorship, of injustice and suppression of free expression in Tunisia," el-Mechri said. "Ghannouchi, you are not alone."
A Tunisian court on Monday sentenced Ghannouchi to a year in prison on terrorism-related charges after he was earlier accused of calling police officers "tyrants."
His Islamist-inspired Ennahdha party condemned the sentence as "an unjust political verdict."
The case was one of several levied by authorities against Ghannouchi, whose party was the largest in parliament before Saied dissolved the chamber in July 2021 as part of a power grab allowing him to rule by decree.
Ghannouchi was detained and remanded in custody in April after remarks warning that eradicating different viewpoints such as the left or political Islam might lead to a "civil war".
Tunisia was the only democracy to emerge from the Arab Spring uprisings in the region more than a decade ago.
His arrest was the latest against Saied's opponents and drew international concern from the European Union, the United States and Germany.
Saied rejected such criticism as "flagrant interference."
In Libya, which has seen more than a decade of stop-start conflict since the 2011 revolt that toppled strongman Moammar Kadhafi, presidential and legislative elections set for December 2021 have still not taken place.
A rival to the senate which el-Mechri heads, the House of Representatives, sits in Libya's east.
His declarations risk creating tensions with Tunisia.
Tripoli-based interim Prime Minister Abdelhamid Dbeibah visited Tunis late last year in a move aimed at warming ties with Saied's government.
Libyan Official Slams Tunisia Jailing Ghannouchi
FILE - Rachid Ghannouchi, head of the moderate Islamist party "Ennahda," is pictured in his office in Tunis, Tunisia, on March 9, 2021.
TRIPOLI - Khaled el-Mechri, president of Libya's Tripoli-based High Council of State, deplored the "return of dictatorship" in neighboring Tunisia, expressing solidarity with Rached Ghannouchi, a jailed opposition leader and high-profile critic of Tunisia President Kais Saied.