A Jewish leader stood before 300 survivors of the Nazis' most notorious death camp and asked world leaders to prevent another Auschwitz.
He warned of a rise of anti-Semitism that has made many Jews fearful of walking the streets, and is causing many 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 a million people murdered at Auschwitz-Birkenau.
He said his speech was shaped by the recent terrorist attacks in France that targeted Jews and newspaper satirists.
"For a time, we thought that the hatred of Jews had finally been eradicated. But slowly the demonisation of Jews started to come back," Mr 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."
The recent attack in Paris, in which four Jews were killed in a kosher supermarket, is not the first deadly attack on Jews in recent years. Last May, a shooting killed four people at the Jewish Museum in Brussels and in 2012 a rabbi and three children were murdered in the French city of Toulouse.
Europe also saw a spasm of anti-Semitism last summer during the war in Gaza, with protests in Paris turning violent and other hostility across the continent.
"This vilification of Israel, the only Jewish state on earth, quickly became an opportunity to attack Jews," Mr Lauder said. "Much of this came from the Middle East, but it has found fertile ground throughout the world."
One Holocaust survivor, Roman Kent, became emotional as he issued a plea to world leaders to remember the atrocities and fight for tolerance.
"We do not want our past to be our children's future," the 85-year-old said to applause, fighting back tears and repeating those words a second time.
US President Barack Obama, who was in Saudi Arabia to pay respects after the death of King Abdullah, issued a statement paying tribute to the six 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 trivialisation of the Holocaust," Mr Obama said.
The commemorations in Poland, which during World War II was under Nazi occupation, 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.
"The survivors are completely gutted that in their lifetime they went through what they went through and that now they are at the end of their life and they don't know what kind of world they are leaving for their grandchildren," said Stephen Smith, executive director of the USC Shoah Foundation.
"That is very disappointing for them. We have let them down."
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.
Among those in attendance were French President Francois Hollande, who has vowed to fight the violent extremism that has wounded his nation, as well as the presidents of Germany and Austria, the perpetrator nations that have spent decades atoning for their sins.
Ukrainian President Petro Poroshenko was also there in a sign of Poland's strong support for Ukraine in its conflict with Russia.