Ex-Iranian President Mohammad Khatami has called the Holocaust a historical reality, clashing with controversial comments by the current president.
Khatami says Iranians live side-by-side with Jews
President Mahmoud Ahmadinejad brought international criticism when he called the Holocaust a "myth" last year.
Mr Khatami said the Holocaust was a "massacre of innocent people, among them many Jews", Iran media reported.
Mr Khatami did not mention the current leader by name, but correspondents say his target was clear.
The former reformist president said the Holocaust was a reality "even if this has been misused and there is enormous pressure on the Palestinian people".
He added: "We should speak out if even a single Jew is killed. Don't forget that one of the crimes of Hitler, Nazism and German National Socialism was the massacre of innocent people, among them many Jews."
Ahmadinejad has taken an ultra-conservative line with the West
Mr Ahmadinejad had said the Holocaust was a "myth" used for the creation of Israel, which should be "wiped off the map".
He said territory should be provided in Germany or Austria to establish Israel "if European countries claim that they have killed Jews in World War II".
The BBC's Sadeq Saba in Tehran says Mr Khatami's comments were clearly directed at President Ahmadinejad.
Mr Khatami tried to maintain a "dialogue between civilisations" during his leadership from 1997 to 2005.
He said on Wednesday: "The persecution of Jews, just like Nazism, is a Western phenomenon. In the East, we have always lived side by side with them. And we follow a religion that states that the death of an innocent person is the death of all of humanity."
Mr Ahmadinejad has taken an ultra-conservative line with the West.
He was criticised on the Holocaust issue two weeks ago by the chairman of Iran's Jewish Council, who said the president's remarks had shocked the international community and caused fear among Iran's 30,000-strong Jewish community.