Michel-Ange Flori, the owner of a French street advertising business, decided to use some of his billboards for what he called an exercise in political satire, posting a picture showing French President Emmanuel Macron dressed like Adolf Hitler.
Macron's personal lawyers and his party have now filed legal complaints alleging that the depictions were a public insult, and Flori said he has been contacted by police acting on the complaint.
The case has turned into a test where France draws the line between freedom of expression and being offensive.
That resonates particularly in a country where the Charlie Hebdo satirical magazine published caricatures of the Prophet Mohammad (PBUH), originally in 2006, that most Muslims view as blasphemous. The French state defended the magazine's right to publish.
"We will not give up on cartoons and drawings, even if others back down," Macron said on Oct. 21 last year in a speech to honour school teacher Samuel Paty, who was killed by a Chechen teenager who wanted to avenge Paty’s use of the cartoons in a class on freedom of expression.
Flori put up the Macron billboards in response to a law adopted by parliament this month barring people from some public venues unless they are fully vaccinated against COVID-19 or can show a fresh negative test.
Some of Macron's opponents say the rules trample on civil liberties and accuse the president of acting like a dictator; the administration argues that it needs to encourage greater vaccination rates.
Flori, whose billboards were posted around his home region in the south of France, said the consensus in his country was on the side of Charlie Hebdo.
"But when it is a matter of making fun of the president by depicting him as a dictator, then it becomes blasphemy, then it is unacceptable," he said in an interview with Reuters, mimicking his critics.
Jean Ennochi, a lawyer for Macron, said the legal complaint was filed for Macron in a personal capacity "because of the offensive nature of the comparison of the President of the Republic with Adolf Hitler".
A representative of Macron's party said it had filed a separate complaint alleging insult and incitement of hatred.
Macron's administration declined to comment.
"I did not expect this at all. That the president would file a complaint against a French citizen," Flori said.
"I caricature," he said. "People may or may not like it but it is all the same, caricature will remain caricature."