Twenty-two editors of leading French media groups published a letter to Alexei Mechkov, the Russian ambassador to Paris, denouncing the detention of Wall Street Journal reporter Evan Gershkovich.

The editors of Le Figaro, Les Echos, Le Monde, Libération and others wrote an open letter expressing their deep concern over the detention of Mr. Gershkovich.

“Keeping Evan Gershkovich in custody is tantamount to taking a hostage,” they wrote. “It is a warning, a threat, even an act of terror directed at the last representatives of international media still present in your country.”

Russia’s main security agency said Thursday that it had detained 31-year-old Mr. Gershkovich while he was on a reporting trip in the city of Yekaterinburg, about 800 miles east of Moscow. Russian authorities have accused him of espionage.

The Journal has vehemently denied the allegations and demanded Mr. Gershkovich’s release.

Last week, more than three dozen top editors of news organizations around the world signed a letter to the Russian ambassador to the U.S. condemning Mr. Gershkovich’s detention. The French editors said in their letter that they sought to support that call for the journalist’s release.

The French editors said the espionage trial facing Mr. Gershkovich would be “a tragic masquerade.”

“The whole array of French media is united in support of Evan and stands with the WSJ to demand his libération,” Bernard-Henri Lévy, president of the supervisory board at broadcaster Arte France, as well as a human-rights activist, wrote in an email.

Earlier on Sunday, U.S. Secretary of State Antony Blinken spoke with Russian Foreign Minister Sergei Lavrov to demand the release of Mr. Gershkovich, as well as another detained American, Paul Whelan.

The following French editors signed the letter:

Dov Alfon, managing editor, Libération
Nicolas Barré, managing editor, Les Echos
Alexis Brézet, managing editor, Le Figaro
Nicolas Beytout, founder and managing editor, L’Opinion
Eric Chol, managing editor, L’Express
Jean-Marie Colombani, founder and publisher, Slate France
Delphine Ernotte, president, France Télévisions; president, European Broadcasting Union
Jérome Fenoglio, publisher, Le Monde
Marc-Olivier Fogiel, managing director, BFMTV
Caroline Fourest, founder and director, Franc-Tireur
Stéphane Gendarme, news editor, M6
Étienne Gernelle, director, Le Point
Vincent Giret, news editor, Radio France
Emmanuel Hoog, publishing director, Les Inrockuptibles
Bernard-Henri Lévy, chairman of the supervisory board, Arte France
Pierre Louette, chief executive, Le Parisien
Frank Moulin, managing editor, RTL 
Véronique Philipponnat, publisher, Elle
Claude Perdriel, publisher, Challenges
Edwy Plenel, publisher, Mediapart
Cécile Prieur, managing director, L’Obs
Riss, publisher, Charlie Hebdo