The world must challenge those who deny the Holocaust happened, says United Nations Secretary General Kofi Annan.
A Warsaw tram bears the Star of David instead of a number
In a statement released to mark Holocaust Memorial Day, Mr Annan described those who questioned whether the Holocaust took place as "bigots".
The German parliament's speaker also said the Nazis' mass murder of Jews should still be commemorated.
Iran's President Mahmoud Ahmadinejad has been widely criticised for claiming that the Holocaust was a "myth".
Holocaust remembrance ceremonies were held across Europe on Friday.
Death camp survivors and religious leaders joined Polish Prime Minister Kazimierz Marcinkiewicz in freezing temperatures for a sombre ceremony at the vast Auschwitz-Birkenau camp in Poland.
The prime minister laid a wreath between the ruins of the gas chambers the Nazis used to murder Europe's Jews during World War II.
Roman Catholic leaders asked Poles to light candles in their windows in remembrance of the six million murdered Jews.
A 1940s red tram marked with the Star of David, like the ones that used to carry Jews in the Warsaw ghetto, once again ran through the streets. But it was empty and nobody got on or off.
In his statement, Mr Annan said "remembering is a necessary rebuke to those who say the Holocaust never happened or has been exaggerated".
"Holocaust denial is the work of bigots, we must reject their false claims whenever, wherever and by whomever they are made."
The UN last year passed a landmark resolution to make 27 January an annual day of remembrance for the Jews murdered during World War II. It was the date 61 years ago when Auschwitz was liberated.
The German parliament speaker, Norbert Lammert, said the last few weeks had shown "not only us Germans how very much we need this day of commemoration".
"It is with consternation that we have realised that even today heads of state describe the Holocaust as a myth and even go as far as making anti-Semitic statements," he said.