BRZEZINKA, Poland — A Jewish leader stood Tuesday before 300 survivors of the Nazis’ most notorious death camp and asked world leaders to prevent another Auschwitz, warning of a rise of anti-Semitism that has caused many Jews to flee Europe.

Ronald Lauder, the president of the World Jewish Congress, made his bleak assessment on the 70th anniversary of the liberation of Auschwitz, speaking next to the gate and the railroad tracks that marked the last journey for more than 1 million people killed at Auschwitz-Birkenau.

He said his speech was shaped by the recent terrorist attacks in France that targeted Jews and satirists.

“For a time, we thought that the hatred of Jews had finally been eradicated. But slowly the demonization of Jews started to come back,” Lauder said. “Once again, young Jewish boys are afraid to wear yarmulkes on the streets of Paris and Budapest and London. Once again, Jewish businesses are targeted. And once again, Jewish families are fleeing Europe.”

President Obama, who was in Saudi Arabia to pay respects after the death of King Abdullah, issued a statement paying tribute to the 6 million Jews and millions of others murdered by the Nazis.

“The recent terrorist attacks in Paris serve as a painful reminder of our obligation to condemn and combat rising anti-Semitism in all its forms, including the denial or trivialization of the Holocaust,” Obama said.

The commemorations in Poland were also marked by a melancholy awareness that it will be the last major anniversary that a significant number of survivors will be strong enough to attend.

Politics also cast a shadow on the event, with Russian President Vladimir Putin absent – even though the Soviet Red Army liberated the camp – the result of the deep chill between the West and Russia over Ukraine.