Dozens of people protested in central Tunis on Friday to demand the release of a journalist and activist detained earlier this week and held on suspicion of "terrorism".
Ghassen Ben Khlifa, the editor of the website Inhiyez and a prominent pro-Palestinian activist, was arrested at his home on Tuesday.
Police officers also searched the house and seized two computers, according to multiple media reports.
Ben Khlifa has since been remanded in custody, said Amira Mohamed, vice-president of the SNJT journalists' union.
"We have had no explanation for why he was arrested," she told AFP.
"The arrest is one of a series of cases against journalists to intimidate them and silence them, which is a serious threat to freedom of the press in the country."
The SNJT organised Friday's demonstration which brought dozens of activists and journalists onto Habib Bourguiba Avenue in central Tunis, under heavy police guard.
They chanted "Free Ghassen!" and "Stop state oppression!", along with slogans against the police and the nearby interior ministry.
Some carried signs supporting press freedom.
Tunisia has seen a spike in prosecutions of journalists, including in military courts, since President Kais Saied seized wide-ranging powers in a dramatic move against parliament last year.
Civil society groups have warned of a return to the methods used by security services under autocrat Zine El Abidine Ben Ali, who was overthrown in the 2011 revolt that sparked the Arab Spring uprisings.
On August 17, a military court sentenced an anti-Saied journalist to three months in prison for public remarks he made about the president and the armed forces.