With its chocolate shops, Trappist beers and gray gloom, Brussels is the center of Europe’s sprawling bureaucracy, a symbol of the loathed policies of austerity. But Brussels also embodies the demographics transforming much of urban Europe, with generations of Muslim immigrants and their descendants now representing roughly a quarter of the population.

The Jewish community is small, about 20,000 people, most of them assimilated, secular Jews like Mr. Mosseri, who usually do not draw attention to their heritage. (A recent report issued jointly by two European Jewish organizations found that 40 percent of European Jews hide their Jewishness.) Now some secular Jews say they have stopped wearing a necklace with the Star of David, or allowing their children to wear T-shirts for a Jewish summer camp on public buses or trains.

And since the start of the conflict in Gaza this summer, many describe social media, especially Facebook, as a swamp of hatred.

“I have friends who are never political and they are posting things about Gaza every day,” said Ms. Frommer, the employee of the nonprofit organization. “It seems like an obsession. Is your obsession because you want to save children, or because you have a problem with Jews?”

In a city so devoted to politics, the issue of Israel can seem unavoidable to some Jews, even those who strive to be apolitical or tend to be critical of Israeli policy. Ms. Frommer grew up in Brussels, but then left for college in Britain, followed by a long stint working in Cambodia. When she returned to Brussels four years ago, she was struck by how much more polarized life seemed. Her Jewish friends were sticking closer together as office chatter now sometimes bore a sharper edge.

This summer, one of her Belgian colleagues repeatedly mentioned the Israeli-Palestinian conflict. “He would often try to bring up the subject when I tried not to,” she said. “Then the subject would shift from Israel to Jews. Then it was, ‘Were there really six million Jews killed in the Second World War?’ ”

Nor was the comment isolated. There have been signs that anti-Jewish sentiment transcended the immediate backlash against the Gaza war. In Hungary, the rise of the far-right Jobbik party has brought concerns that anti-Semitic views are gaining mainstream traction.