Dozens of world leaders gathered in Jerusalem to mark the 75th anniversary of the liberation of Auschwitz, amid a backdrop of rising anti-Semitism in Europe and the United States.
Israel has hailed the World Holocaust Forum at the Yad Vashem memorial centre as the biggest international gathering in its history.
Russian President Vladimir Putin, meeting on the sidelines of the conference with his Israeli counterpart Reuven Rivlin, said xenophobia and anti-Semitism must be opposed everywhere, regardless of who was behind the hatred.
"You just said that it's not known where anti-Semitism ends," Mr Putin told Mr Rivlin, referring to remarks the Israeli president made at their meeting.
"Unfortunately we do know this- Auschwitz is its end-result."
The high-profile guest list for today's commemoration includes US Vice President Mike Pence, French President Emmanuel Macron, German President Frank-Walter Steinmeier and Britain's Prince Charles.
Mr Steinmeier will be the first German president to deliver a speech at Yad Vashem. He planned to address the forum in English out of consideration for Holocaust victims who might be distressed at hearing German spoken at the event, his office said.
Watch: Drone footage of Auschwitz-Birkenau death camp
Israeli Prime Minister Benjamin Netanyahu called Iran "the most antisemiti cregime on the planet" during an address at the Holocaust memorial in Jerusalem.
"I am concerned that we have yet to see a unified and resolute stance against the most anti-Semitic regime on the planet. A regime that openly seeks to develop nuclear weapons and annihilate the one and only Jewish state," Mr Netanyahu told assembled world leaders.
"Israel salutes President [Donald] Trump and Vice President Pence for confronting the tyrants of Tehran," he told the audience, which included Mr Pence.
A global survey by the US-based Anti-Defamation League in November found that global anti-Semitic attitudes had increased, and significantly so in Eastern and Central Europe. It found that large percentages of people in many European countries think Jews talk too much about the Holocaust.
More than one million people, most of them Jews, were killed at the Auschwitz-Birkenau camp during World War II.
Overall, some six million Jews died in the Holocaust.
However, the president of Poland, where the death camp was built by the Nazi German occupiers during World War II, will stay away due to disputes with both Russia and Israel.
Polish President Andrzej Duda turned down an invitation to the conference, expressing dissatisfaction that representatives of Russia, France, Britain, the United States and Germany would speak, while Poland was told it would not be allowed to.
Polish leaders have also been angered by comments made by Mr Putin last month suggesting Poland shared responsibility for the war.
Poland, which was invaded first by Nazi Germany and then by Soviet forces in September, 1939, sees itself as a major victim of the war, in which it lost a fifth of its population.
Mr Putin also criticised those who try to rewrite history, though he made no reference to Poland, while unveiling a monument to the residents and defenders of Nazi-besieged Leningrad, Russia's second city now renamed St Petersburg.
"Here, as in Russia, people are concerned, alarmed and outraged by attempts to deny the Holocaust and to revise the results of World War II, to whitewash murderers and criminals," Mr Putin said.
Speeches at Yad Vashem are likely to focus on the horrors of the Nazi Holocaust as well as on the recent rise in anti-Semitism rhetoric and attacks worldwide.
Poland will host its own ceremony at the Auschwitz-Birkenau Memorial and Museum on 17 January, as it does every year.