In the aftermath of a second U.S. synagogue shooting in six months, Jewish leaders around the world urged a "wake-up call" on the threat of anti-Semitism.

On the last day of Passover Saturday, a gunman walked into the Chabad of Poway Synagogue near San Diego and opened fire, leaving one woman dead and three others injured.

The attack came exactly six months after a man fatally shot 11 people at the Tree of Life synagogue in Pittsburgh — in the deadliest attack against Jews in American history. The suspect in that attack, who has been indicted on a slew of hate crime charges, had a history of posting anti-Semitic messages online.

While police are still establishing the motive of Saturday's shooting, an anti-Semitic open letter posted under the name of a man suspected of opening fire at the synagogue was left on a far-right message board hours before the attack.

The violence sparked a renewed cry for action on anti-Semitism.

The director of the U.S. Holocaust Memorial Museum in Washington said the organization was "alarmed" by the second such incident in six months.

"Moving forward this must serve as yet another wake-up call that anti-Semitism is a growing and deadly menace,” Sara J. Bloomfield said in a statement.

Israel's Prime Minister Benjamin Netanyahu called the shooting "an attack on the heart of the Jewish people" and urged the international community to step up the struggle against anti-Semitism.

He vowed to convene a special discussion next week in light of the upsurge in anti-Semitic attacks around the world.

David Hirsh, a lecturer in sociology at London's Goldsmiths College, told NBC News anti-Semitic sentiment has never gone away.

"It sits there in our culture and we think it is a thing of the past, too vulgar and awful to constitute a contemporary threat. But anti-Semitic ways of thinking are nevertheless entrenched in our subconscious," he said.

Yad Vashem, the Jerusalem-based World Holocaust Remembrance Center, said: "As we approach Holocaust Remembrance Day, which is dedicated to commemorating the memory of the six million men, women and children murdered for being Jewish, we shall gravely consider the dangers of unchecked anti-Semitism."

"In recent months, we have witnessed a distressing resurgence in incidents of anti-Semitic attacks," said chairman Avner Shalev. "On every continent, violence against Jews, merely because they are Jews, occurs. The time has come for world leaders to speak out and condemn anti-Semitism in all its expressions."

Saturday's shooting marks the latest tragedy in a string of attacks on houses of worship around the world.

The Anti-Defamation League (ADL), a New York-based organization dedicated to the fight against anti-Semitism, called the shooting "an attack on the entire Jewish community" and a "brutal hate crime."

"​This shooting is a reminder of the reality and virulence of anti-Semitism," said CEO Jonathan Greenblatt. "From Charleston to Pittsburgh to Oak Creek and from Christchurch to Sri Lanka, and now Poway, we need to say 'enough is enough.'"

Britain's foremost Jewish leader likewise urged a defiant response. "As despicable attacks on innocent people at prayer become more frequent, whether in Sri Lanka, Christchurch or Pittsburgh, we will not be timid nor passive in our grief," said Chief Rabbi Ephraim Mirvis.

"We will fiercely challenge those who promote or excuse a narrative of demonization and we will be proud and unapologetic about our faith and the values we hold dear," he added.

The World Jewish Congress, an international organization that represents Jewish communities and organizations in one hundred countries around the world, said it was horrified by the attack.

"There is absolutely no justification or explanation for such violence, and it is inconceivable that yet again innocent people have been targeted simply for their religion and for choosing to attend a place of worship,” said the group's president Ronald S. Lauder in a statement.

"People of all faiths must stand together and declare that we will never tolerate such hatred,” Lauder said.