He singled out for his insults the local gendarmes at Faou, in Brittany, but the officers came upon the remarks, which had been online for three months.
The court ruled that the remarks constituted a "public outrage", and as well as handing down the three-month jail sentence, ordered their author to pay 1,200 euros in damages to the officers concerned.
The defendant was not in court for the hearing.
Internet sans Frontieres (Interet Without Borders), which campaigns for free speech online, denounced the severity of the sentence.
This was the first time that someone had been handed a jail sentence for this kind of offence, it said.
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The court ruled that the remarks constituted a "public outrage", and as well as handing down the three-month jail sentence, ordered their author to pay 1,200 euros in damages to the officers concerned.
The defendant was not in court for the hearing.
Internet sans Frontieres (Interet Without Borders), which campaigns for free speech online, denounced the severity of the sentence.
This was the first time that someone had been handed a jail sentence for this kind of offence, it said.
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