PARIS — A French court has convicted a man for carrying a weapon at an illegal demonstration in front of the U.S. Embassy protesting a video that protesting Muslims say insulted the Prophet Muhammad.

The 24-year-old convert to Islam was sentenced to three months in prison. Saturday's ruling came hours after police detained a man in the western city of La Rochelle suspected of threatening to decapitate the editor of a French satirical weekly that published lewd caricatures of the prophet on Wednesday amid protests around the world against the amateurish film produced in California.

The swift action in both cases reflects concern in France, where Islam is the second biggest religion after Christianity, about potential fallout from the video and the caricatures. Protests planned for Saturday were banned, and police increased security around the U.S. Embassy, at the main Paris mosque and at other sensitive sites.

A week ago, police detained 151 protesters who suddenly gathered at the U.S. Embassy without authorization and eventually released all but the convicted man, Loic Guibet. Police found a retractable club bearing his fingerprints in a garbage can nearby. Guibet, who works for the French railway and is married, claimed he brought the weapons "preventatively" should a Zionist group blamed for violence in the past show up.

The Sipa news agency quoted him as saying that he "didn't come with the goal of picking a fight."

Guibet was allowed to go free after the sentencing and it's possible he will serve no time at all, which is not unusual in France for prison sentences of less than two years. A judge will decide whether he goes to jail, does community work or wears an electronic bracelet.

Meanwhile, in La Rochelle, police detained a 43-year-old man for questioning after the prosecutor's office opened a probe over a threat, made in an Internet forum against the editor-in-chief of Charlie Hebdo magazine, Sipa reported.

Wednesday's publication of the cartoons by Charlie Hebdo raised concerns that French interests worldwide could face violent protests like the ones targeting the U.S. over the anti-Islam video that mocked Muhammad. France ordered around 20 overseas missions closed Friday, the Muslim holy day, later extending the closures until Monday.

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  • Publishing director of the satyric weekly Charlie Hebdo, Charb, holds the newspaper as he talks to the media in Paris, on Wednesday, Sept. 19, 2012. (AP Photo/MIchel Euler)

  • Publishing director of the satyric weekly Charlie Hebdo, Charb, gestures as he talks to the media in Paris, on Wednesday, Sept. 19, 2012. (AP Photo/MIchel Euler)

  • Publishing director of the satyric weekly Charlie Hebdo, Charb, displays the front page of the newspaper as he poses for photographers in Paris, on Wednesday, Sept. 19, 2012. (AP Photo/MIchel Euler)

  • A French policeman stands guard outside the headquarters of the satirical weekly Charlie Hebdo in Paris, on Wednesday, Sept. 19, 2012. (AP Photo/MIchel Euler)

  • Satirical French newspaper Charlie Hebdo's editor in chief, Charb, answers reporters in front of the headquarters of the newspaper, that "invited" the Prophet Muhammad as a guest editor this week, in Paris, Nov. 2, 2011. (AP Photo/Thibault Camus)

  • A police officer stands in front of the headquarters of satiric French newspaper Charlie Hebdo, that "invited" the Prophet Muhammad as a guest editor this week, in Paris, Wednesday, Nov. 2, 2011. (AP Photo/Thibault Camus)

  • A police officer stands front of the headquarters of satiric French newspaper Charlie Hebdo, that "invited" the Prophet Muhammad as a guest editor this week, in Paris, Wednesday, Nov. 2, 2011. A police official said the fire broke out overnight at the offices of Charlie Hebdo, and the cause remains unclear. No injuries were reported. The official spoke on condition of anonymity because an investigation into the fire is under way. (AP Photo/Thibault Camus)