The Prince of Wales has warned "hatred and intolerance still lurk in the human heart", at an event in Israel marking 75 years since the liberation of the Nazi death camp at Auschwitz.
During his speech in Jerusalem, he said lessons of the Holocaust are still "searingly relevant" and called on world leaders to be "fearless in confronting falsehoods" and violence.
About 40 leaders attended the event.
The Nazis murdered more than a million people at Auschwitz, most of them Jews.
Prince Charles delivered his call for action at the World Holocaust Forum being staged at Yad Vashem, the World Holocaust Remembrance Centre.
Russian President Vladimir Putin, French President Emmanuel Macron and US Vice-President Mike Pence were among those attending.
However, a decision by Poland's President Andrzej Duda not to join them threatened to overshadow the event.
Prince Charles, on his first official trip to Israel and the occupied West Bank, told them that hatred and intolerance "tell new lies, adopt new disguises, and still seek new victims".
"All too often, language is used which turns disagreement into dehumanisation," he said.
"Words are used as badges of shame to mark others as enemies, to brand those who are different as somehow deviant.
"All too often, virtue seems to be sought through verbal violence. All too often, real violence ensues, and acts of unspeakable cruelty are still perpetrated around the world against people for reasons of their religion, their race or their beliefs.
"Knowing, as we do, the darkness to which such behaviour leads, we must be vigilant in discerning these ever-changing threats; we must be fearless in confronting falsehoods and resolute in resisting words and acts of violence.
"And we must never rest in seeking to create mutual understanding and respect."
The focus, say the organisers, will be on fighting anti-Semitism today.
But some speeches - particularly those outside of the event - look set to go further; as Jerusalem bristles with presidents and princes in what officials say amounts to the biggest political gathering since Israel's founding.
Prime Minister Benjamin Netanyahu has already used the lead up to say the number one lesson from Auschwitz is stopping a nuclear armed Iran.
While the decision to give the podium to President Putin of Russia has sparked fury in Poland.
Its nationalist president Andrzej Duda is staying away in protest at not being invited to speak; accusing Mr Putin of distorting the history of the Holocaust and the war to attack his country.
Ahead of the event, the prince met survivors of the Holocaust, saw the ancient Dead Sea Scrolls, and visited Israel's President Reuven Rivlin.
Mr Rivlin told the prince that Israel "deeply appreciates" his attendance at the gathering, which he said would help to "show that when we are united we can fight this phenomenon".
He also told the prince that "we still expect your mother to come" to Israel. The Queen has never visited the country during her 67-year reign.
To commemorate the visit, Charles was invited to plant an oak tree in the gardens of the president's official residence, Beit HaNassi.
During his two-day trip, Prince Charles is also likely to visit the grave of his grandmother, Princess Alice of Battenberg, in Jerusalem's Church of St Mary Magdalene.
She was honoured by the Jewish people for hiding and saving the lives of Jews in Nazi-occupied Athens, Greece, during World War Two.
In his address on Thursday, Prince Charles spoke of his "immense pride" at the honour, saying he has "long drawn inspiration from the selfless actions of my dear grandmother".