In a joint statement Thursday, some 66 signatories expressed their "full support for the UGTT, which has been methodically targeted by the authorities which consider it to be the last obstacle in their way of seizing all powers".
They slammed "desperate attempts to criminalize union work," pointing their fingers at President Saied and his alleged heavy hand.
They also accused authorities of seeking to "tighten the grip on anything related to public affairs", adding such moves paved the way for "repression, tyranny and individual domination."
The position statement was signed by the Tunisian Communist Party, the Tunisian Association for Rights and Freedoms (ADL) and key figures such as philosopher and anthropologist Youssef Seddik and activist Bochra Belhaj Hmida, among others.
Following Kaabi's arrest, UGTT chief Noureddine Taboubi had accused Saied of "trying to divert attention" from the election result and "the utter failure of his economic and social decisions."
Kaabi is set to face a court hearing on February 23 on charges of causing financial losses to a state-owned company after launching the toll barrier workers' strike.
Just over 11 percent of Tunisians voted last month in the second round of legislative elections that were seen as the final pillar of Saied's overhaul of the country's post-revolution political system.
The UGTT has announced that it will hold sit-ins over the next weeks to protest Saied's policies, culminating in a rally in the capital on March 11.