Police forces face demonstrators during a rally in Tunis. AP
The arrest of Anis Kaabi, the UGTT's top official for highway workers, was "a blow to union work and a violation of union rights as well as international treaties", it said in a statement.
Saied has been accused of using the judiciary to silence his political opponents since he launched a dramatic power grab in July 2021.
The North African country is grappling with a deepening economic crisis and has seen a string of strikes in recent weeks, hitting the education and public transport sectors.
Earlier this week, workers at toll barriers on Tunisian highways went on strike for better pay, meaning tolls went uncollected for two days.
In its statement, the union called for Kaabi's immediate release, recalling the constitutional right to strike and saying his arrest had traumatized his family.
Kaabi's arrest came hours after Saied threatened "those who block roads", saying they "cannot remain unaccountable".
In a speech at a National Guard base in the capital Tunis, Saied said that "union rights are guaranteed under the constitution, but constitutional rights cannot be exploited for political ends".
The UGTT insisted that the strike was legal, and said Kaabi was facing prosecution for causing financial losses to the publicly-owned company managing the barriers, and thus to the treasury.
The UGTT, which jointly won the Nobel Peace Prize in 2015 for its role in the country's democratic transition, has around a million members working in every sector of the Tunisian economy.