BEIRUT — An American journalist has been detained in Lebanon on suspicion of sending footage of anti-government protests to an Israeli news outlet — a potentially serious crime under Lebanese law — authorities here said Monday.

Journalist advocacy groups and friends of freelancer Nicholas Frakes, 24, said they believed his arrest in downtown Beirut on Sunday night was a case of mistaken identity stemming from a misunderstanding about the way international news coverage works.

Frakes was picked up by security forces after live footage streamed from the scene of the protests was broadcast on the Facebook page of the Israeli newspaper Haaretz, Lebanese authorities said. Security officials traced the location from which the feed was filmed, found Frakes at the scene and detained him, Lebanon’s General Security Directorate said in a statement Monday night.

Haaretz said Monday that it had taken the video from the Reuters news agency, which distributes such media to clients around the world. Reuters issued a statement denying that Frakes had been involved in the filming of the video.

But Lebanese officials said Frakes was still being interrogated late Monday, after a judge referred his case to the military intelligence department responsible for investigating illegal contacts between people in Lebanon and Israel.

Lebanon has been in a state of war with Israel since 1948, and interactions between the two countries are forbidden.

Ignacio Miguel Delgado, the Middle East and North Africa director for the Committee to Protect Journalists, urged the authorities to release the journalist.

“It’s a misunderstanding,” he said. Frakes twice telephoned a friend in the hours after his detention and said he was fine, Delgado and a friend said. But he has not been heard from since Monday morning and concern is growing about his welfare, Delgado said.

Tensions in Beirut have increased as a three-month-old protest movement demanding significant changes to Lebanon’s system of governance increasingly turns violent. Journalists have been beaten and detained, raising concerns that the authorities are seeking to deter coverage of the protests, the Committee to Protect Journalists said in a statement last week.

More than 500 people were hurt in clashes between protesters and police in downtown Beirut over the weekend. They including two Lebanese journalists injured by rubber bullets, according to civil defense agencies. Journalists have been briefly detained and beaten, the Committee to Protect Journalists said.

“Journalists in Lebanon should be able to cover protests without the fear that they will be detained or attacked by a police officer,” the organization said in a statement.

Frakes says on his Facebook page that he is from Rancho Cucamonga, Calif., and that he studied journalism at Britain’s Brunel University. He arrived in Beirut about a year ago to launch a career as a freelance journalist, according to friends, and had been writing for the New Arab online news outlet. The New Arab said Frakes was simply “at the wrong place at the wrong time” and urged his release.

Haidamous reported from Washington.