A Tunisian military court sentenced blogger Yassine Ayari to a year in prison Tuesday for defaming the military, in a case that has been criticised by human rights groups.

The 33-year-old was arrested on December 25 on his return from Paris, following an initial three-year sentence handed down the previous month in his absence.

Ayari told the court before the new ruling that the charges were a "settling of scores against me for criticising officers in the army."

He had accused officers and defence ministry officials of financial abuse.

Dozens of protesters gathered outside the court demanding Ayari's immediate release, saying the conviction was a violation of freedom of expression.

The demonstrators chanted "Down with military trials" and "No return to dictatorship".

The defence team said it planned to appeal.

"Freedom of expression is the only benefit of the revolution and today we see a blogger sentenced harshly by a military court for criticising the army," said lawyer Malek Ben Amor.

Ayari was also an activist during the regime of dictator Zine El Abidine Ben Ali, who was ousted in a 2011 uprising that triggered the Arab Spring revolts.

In recent months, he had published blogs critical of the Nidaa Tounes party, which won Tunisia's first-post revolution parliamentary elections in October.

New York-based Human Rights Watch has described the case against him as "not worthy of the new Tunisia".

It has urged parliament to reform laws that lead to imprisonment for defaming or insulting state institutions, and to remove jurisdiction of military courts over civilians.