Mayor Bill de Blasio decried a worldwide “anti-Semitism crisis” in a speech Saturday at a Manhattan synagogue.

“There is an anti-Semitism crisis. It is not by any means just in this city, and in this country, but more and more in the entire Western world,” he told a packed congregation at the Ansche Chesed Synagogue on West 100th Street, a conservative temple.

The mayor singled out young people for the recent spate of violent attacks against Jews.

“As we look across this city, there are too many young people who harbor hateful views. We have to find out why that is happening and we have to stop it,” he said, citing social media as a key source of the bias.

As the mayor walked into the house of worship, wearing a black yarmulke, a member with two young daughters yelled out, “You’re the worst mayor in the world. The worst mayor in the world. The crime is outrageous. The crime in this neighborhood is horrible, all because of you.”

De Blasio glanced at the man but did not address him. The man later declined to give his name to reporters.

Inside, de Blasio made a brief speech during the service, attended by about 500 congregants.

“What we see is not just a series of painful incidents, inappropriate incidents, violent incidents, clearly anti-Semitic incidents. These are not isolated,” the mayor said, referring to the violence against Jew. “We all know we’re talking about the physical health, the mental health of society as a whole. The first step is to acknowledge it.”

He added, “There has been a raging debate in these weeks of what are the steps that would work. Some focus on the immediate question of safety and on the role that our security forces play. They will say it’s not enough. I agree that’s not enough. I also know that this crisis can not be confronted without an assertive and aggressive security presence throughout the Jewish communities.”

He also called on communities working together in safety coalitions and patrols with “members of all different backgrounds, visibly together on the streets, particularly in communities most afflicted by these incidences, especially in Brooklyn.”

In addition, schools should focus lessons on the impact of hate crime, he said.

“We’ve had a curriculum for a long time that focuses on respect and understanding and tolerance. Now it needs to be deepened and focus on the impact of hate crimes, particularly the growing anti-Semitism around us.”

The mayor’s speech came ahead of a planned Sunday anti-Semitism “Solidarity March” in Lower Manhattan, during which demonstrators will march over the Brooklyn Bridge.