Sandor Lezsak, also a deputy speaker of the Hungarian parliament, is scheduled to give a speech after a mass in Budapest Saturday in honour of the 150th anniversary of the birth of Miklos Horthy.
Horthy, an autocrat who ruled Hungary from 1920 to 1944, passed anti-Jewish laws and oversaw the deportations of several hundred thousand Hungarian Jews to Nazi German death camps.
Since 2005 the event's date, January 27, is a UN- designated Holocaust memorial day marking the 1945 liberation of Auschwitz-Birkenau, the largest of the camps.
An estimated total of 600,000 Hungarian Jews perished during the Holocaust.
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In an open letter addressed to Lezsak published on the group's website, Mazsihisz head Andras Heisler said the official's participation in the Horthy event on the UN Holocaust day "tramples on the memory of all the Hungarian victims".
"It can only amount to the falsification of history,... no state representative should contribute to the building the cult of Horthy," said Heisler.
Heisler also criticised the event organisers, the Association of Christian Professionals group, as Horthy's birthday is June 18, not January 27.
The late leader is revered by far-right groups and some public figures as a hero for opposing a short-lived communist revolution in 1919 and restoring some of the territory lost by Hungary at the 1920 Trianon Peace Treaty.
Last year Prime Minister Viktor Orban called Horthy an "exceptional statesman" in the period after World War I.
Orban has been accused of tacitly encouraging efforts to rehabilitate the leader and other controversial figures from the inter-war period, though he has said his government has a policy of "zero anti-Semitism".