The detention fuels concerns over free speech in Tunisia since Saied seized extra powers in 2021, moving to rule by decree and then assuming authority over the judiciary.
"Heni was interrogated in the absence of lawyers. What happened is a farce that enhances the dictatorial approach," Heni's lawyer, Dalila ben Mbarek, told Reuters.
Heni, the presenter of a daily radio programme, has not commented on the allegations of insulting the president, which carry a maximum penalty of five years in prison.
Judges have detained or opened investigations into more than 20 political, judicial, media and business figures with opposition ties over recent months, accusing some of plotting against state security.
The main opposition parties have decried the arrests as politically motivated and rights groups have urged authorities to free those detained.
Saied has described the detainees as terrorists, criminals and traitors, and says judges who free them would be abetting their alleged crimes.
Free speech is one of the key reforms that Tunisians won after the 2011 revolution that toppled dictatorial President Zine El Abidine Ben Ali. But activists, journalists and politicians say this freedom is once again under threat.
Saied rejects accusations of targeting freedoms and says he will not be a dictator.
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