Saied has pushed through sweeping changes to the political system in the sole democracy to have emerged from the Arab Spring uprisings, concentrating near-total power in his office since he froze parliament and sacked the government in July 2021, AFP reported.
In the biggest crackdown since the president's power grab, police have arrested around 20 prominent political figures over the past two weeks, primarily Saied's opponents.
"Freedom, freedom, down with the police state," demonstrators chanted as they marched in Tunis on Saturday, also calling for "a halt to impoverishment" in the North African country.
UGTT chief Noureddine Taboubi accused the president of targeting the powerful union as part of a wider crackdown against critics.
Taboubi condemned the latest wave of arrests and the imprisonment since February of Anis Kaabi, a top UGTT official for highway workers, who had been detained after a strike by toll barrier employees.
"We will never accept such arrests," Taboubi told the protesters.
The UGTT has around one million members and shared a Nobel Peace Prize in 2015 with three other civil society groups for promoting national dialogue in the country of about 12 million inhabitants.
Taboubi called on Saied to embrace "dialogue" and "democratic" ways, slamming the president for pursuing a "violent discourse... that is dividing the country".
The UGTT chief also defended "the rights of migrants, regardless of their nationality or the colour of their skin".
"Tunisia is a country of tolerance, no to racism," he told the crowd.
Saied last month ordered officials to take "urgent measures" to tackle irregular migration, claiming without evidence that "a criminal plot" was underway "to change Tunisia's demographic make-up".
The rally on Saturday came as some 300 West African migrants in Tunisia prepared to be repatriated, fearful of a wave of violence since Saied's comments.