Jewish kippas, or skullcaps, are seen on display at a store in downtown west Jerusalem, Israel, Friday. Getty Images

People across the world posted photos of themselves wearing Jewish skullcaps on social media Friday as part of Everyone With A Kippa Day. The unofficial observance, marked with the hashtag #TousAvecUneKippa, was intended to show solidarity with the French Jews, who were warned this week that it might not be safe for them to advertise their faith, the Jewish Press reported. Kippa are also called yarmulkes.

"The idea is that everybody — Jewish or not — should wear a kippa, because if everybody wears one, nobody is a target anymore," the initiative's co-creator, Sophie Taieb, told BBC News. She added that in addition to people wearing the skullcaps, she'd seen snapshots of cats and Photoshopped pictures of the Mona Lisa donning them on the Internet Friday.

Everyone With A Kippa Day came four days after a 15-year-old claiming allegiance to the Islamic State group attacked Jewish teacher Benjamin Amsellem with a machete in Marseille, France. Amsellem was traveling to work, wearing a kippa and holding a Torah book at the time. He had non-life-threatening injuries, but French Jewish politician Alain Ghozland was found dead the same day.

The two incidents inspired Jewish leader Zvi Ammar to urge men and boys to skip wearing the kippa caps "until better days." "Unfortunately for us, we are targeted. As soon as we are identified as Jewish we can be assaulted and even risk death," Agence France-Presse reported he said.

France's 600,000 Jewish people mean the country has the third-largest Jewish population in the world, according to NPR. But anti-Semitism was on the rise. A recent study found that 63 percent of French Jews said they'd been insulted for their religion, and 2014 statistics from the interior ministry reportedly indicated that half of all hate crimes in the country were on Jewish people.