Holocaust Day marked at Nazi death camp Auschwitz
Auschwitz survivors and Israeli Prime Minister Benjamin Netanyahu are among those gathering in Poland, where the camp was built under German occupation.
In Berlin, Israeli President Shimon Peres urged Germany and other countries to pursue Holocaust perpetrators.
More than a million people were murdered by the Nazis at Auschwitz.
The great majority were Jews but they also included Poles, Roma Gypsies and Soviet prisoners of war.
The camp was liberated by the Soviet Red Army on 27 January 1945.
Peres plea
At least six million Jews were killed by the Nazis during World War II.
Addressing Germany's parliament, Israel's president Shimon Peres said some of those who carried out the Holocaust "still live on German and European soil, and in other parts of the world".
"My request of you is: Please do everything to bring them to justice."
He also recalled leaving his grandfather behind in Poland, when his family moved to Palestine in 1934. His grandfather was later killed by the Nazis - herded into a synagogue with the other Jews of his village, and burned to death.
"I remember his poignant embrace. I remember the last words and the order I heard from his mouth: 'My boy, always remain a Jew'," he said.
Replica sign
Some of those who survived the Holocaust gathered at the site of the Auschwitz and neighbouring Birkenau death camps on Wednesday, despite the cold and the snow.
Many had relatives with them.
They passed beneath the notorious sign above the entrance, reading "Arbeit Macht Frei", or "Work Makes You Free".
The sign is a replica. The original was stolen last month. It has been recovered, in three pieces, but not yet repaired and repositioned.
Later Mr Netanyahu was to speak at a commemorative ceremony.
Poland's President, Lech Kaczynski, was also expected and US President Barack Obama was sending a video message.
There has been some controversy over the presence of an Israeli Arab MP, Mohammed Barakeh, in Mr Netanyahu's delegation.
Some Palestinians have criticised him for sympathising with Israel at a time when many Palestinians are suffering.
But Mr Barakeh is expected to highlight the Palestinian plight and condemn Israeli policy - drawing condemnation from some Israelis.