The president was speaking on the eve of Remembrance Day, which commemorates gruesome massacres carried out between 1943 and 1945 on Italy's north-eastern border.
"Remembrance day marks an agonising chapter for our country [...] a tragedy caused by a calculated initiative to purge on an ethnic and nationalist basis," he said. "The massacres, violence and suffering endured [...] cannot be forgotten, minimized or suppressed."
Between 1943 and 1945, Yugoslav troops massacred thousands of Italians, throwing their victims dead or alive into 'foibes', deep stone cavities, typical of the border region with Slovenia and Croatia. This political and ethnic cleansing known as the "Foibe massacres" is commemorated every year on February 10th.
These events "make us aware of the grave risks of extreme nationalism, ethnic hatred and systematic violence spurred by ideology," said Mattarella.
As the election race heats up ahead of the March 4th vote, racism and immigration have been placed at the heart of the debate.
Since 2014, more than 630,000 migrants have arrived on Italian shores.
On Saturday a right-wing sympathizer embarked on a racially-motivated shooting spree injuring several people from African countries in the small, central city of Macerata.
Luca Traini, 28, said he wanted to avenge the death of an Italian woman whose body was found cut into pieces, allegedly by a Nigerian asylum seeker.
Clashes broke out on Thursday in Macerata between the police and supporters of the right-wing group Forza Nuova out to show their support for Traini's actions.
Several pro-migration associations have called for peaceful counter-demonstrations to take place in Macerata this Saturday.