A union official, who asked not to be named, told Reuters that its general secretary for culture, Abdel Nasser Ben Amara, was arrested on Wednesday after a protest at the Culture Ministry on Monday. He gave no more details.
Government officials were unavailable for comment.
Another senior union official was detained last month for organizing a strike by highway tollbooth operators, prompting the UGTT's newspaper to accuse Saied of declaring war on the organization and its 1 million members.
The UGTT has been holding rallies around Tunisia since last month after weeks of arrests in the biggest crackdown since Saied began ruling by decree in 2021.
Police have detained more than a dozen prominent opposition figures, accusing them of conspiring against state security.
Critics say Saied has dismantled the democracy and freedoms won in a 2011 revolution that triggered the Arab Spring.
He counters that he is saving Tunisia from chaos.