Noemi Di Segni, the president of the Union of the Italian Jewish Community, Italy’s largest Jewish organization, said the country was experiencing a “rise in anti-Semitism, which manifests itself in many ways.”

She said an increased tolerance for Mussolini and Fascist nostalgia was troubling and “creating an uncomfortable climate for Jews.” The police protection of Ms. Segre, she said, was not only to protect the senator, “it’s about protecting Italy.”

Politicians and Jewish groups have organized rallies in Milan and Rome to show solidarity with Ms. Segre. She has received calls of support, her son said, from Prime Minister Giuseppe Conte and President Sergio Mattarella.

In 2018, Mr. Mattarella made Ms. Segre, who had never worked in politics, a senator for life, in part because of her years giving students a firsthand account of the Holocaust. She said at the time she felt it her obligation to “pass on the memory.”

But around that time Mr. Belli Paci said he and his brother noticed a spike in anti-Semitic insults, some wishing their mother death, on the web. While they did not inform their mother, they reported the messages to the counterterrorism police, he said.

That vitriol is not unique to Ms. Segre.

While speaking at a conference three weeks ago in Milan, Betti Guetta, an anti-Semitism researcher at the Center of Contemporary Jewish Documentation, said that her group had noticed that general anti-Semitic offenses, mostly online, had increased in recent years. They had reached the clip of nearly 200 a day, she said, later calling the statistic an estimate.

At the conference, she said, she also noted that even Ms. Segre was a frequent target of attacks.

The figure of 200 daily attacks was subsequently misinterpreted as referring to aggression specific to Ms. Segre. Then, on Tuesday, Forza Nuova, a neo-Fascist political party, raised a banner against anti-Fascists near a theater where Ms. Segre was scheduled to speak.