Iranian threats loom large over day's sadness.
The international community commemorated International Holocaust Remembrance Day Friday, with world leaders vowing not to allow such a catastrophe to happen again.
"I join people here at home, in Israel, and around the world in commemorating International Holocaust Remembrance Day, as we mark one of the darkest, most destructive periods in human history," U.S. President Barack Obama said in a statement.
"We are reminded to remain ever-vigilant against the possibility of genocide, and to ensure that ‘Never Again’ is not just a phrase but a principled cause. And we resolve to stand up against prejudice, stereotyping, and violence – including the scourge of anti-Semitism – around the globe."
Some six million Jews were murdered by the Nazis during the Holocaust.
United Nations Secretary General Ban Ki Moon called on the international community to learn the lessons of history.
"Today, as we remember all those lost during the Holocaust – young and old alike -- I call on all nations to protect the most vulnerable, regardless of race, color, gender or religious beliefs."
In Israel, the day was marked against the backdrop of an Iranian regime that has working to develop nuclear weapons and has frequently threatened to wipe Israel off the map.
"I want to remind everyone of the main lesson of the Holocaust against our people - that ultimately when there is threat to our existence, we must not leave our fate in the hands of others," said Israeli Prime Minister Benjamin Netanyahu.
"When it is a question of our fate, it is our obligation to rely only on ourselves."
Israel's own day of commemoration, Yom Hashoah, occurs later in the spring.
