French President Emmanuel Macron has denounced the "unspeakable" defacing of a Nazi massacre memorial, adding that everything would be done to find the people behind the act.

The main entrance sign for the memorial at Oradour-sur-Glane in central France was vandalized with the word "martyr" crossed out with white paint. Slogans denying the massacre and the word "menteur" or "liar" had also been added to the sign.

The graffiti was discovered when the memorial center opened on Friday morning, according to its president, Fabrice Escure. 

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"Everything will be done to ensure that the authors of this are brought to justice," Macron said in a statement. 

Politicians across the board have also condemned the act.

"To violate this place of reflection is also to violate the memory of our martyrs," said Prime Minister Jean Castex.

Some 642 men, women and children were killed on June 10, 1944 by a German SS division at Oradour-sur-Glane. It was one of the worst single massacres by the Nazis in France during World War II.

France has seen repeated acts of vandalism on Jewish cemeteries over the last few months, triggering concerns over remembering World War II.

dvv/nm (AFP, Reuters)