Yassine Ayari, 33, was taken into custody overnight after landing at Tunis-Carthage airport and was whisked to jail in line with the court verdict pronounced against him.
The military prosecutor said Ayari had been sentenced in absentia on November 18 to three years' jail on charges of having "defamed army officers and senior defence ministry officials".
Ayari, who was returning from France when he was detained overnight, had accused these officials of financial abuse, the prosecutor said in a statement.
Ayari, who has posted on Facebook that he stands by every word he has written, has appealed the charges against him and a new hearing in the case has been set for January 6, Ben Amor said.
Also Read
"This case undermines freedom of speech and is the first of its kind to emerge under the reign of Beji Caid Essebsi," said the lawyer, adding that "Yassine Ayari is paying the price of his political views".
His victory follows October legislative polls won by his Nidaa Tounes party.
In recent months, Ayari had published blogs very critical of Nidaa Tounes.
Essebsi is a veteran politician who served under previous Tunisian regimes, including in the government of autocratic president Zine El Abidine Ben Ali who was ousted in the 2011 uprising.