A prominent public art installation in Vienna featuring large portraits of Holocaust survivors was badly damaged overnight on Sunday after an unknown assailant slashed the pictures with a knife.

Photographs published on Monday morning showed many of the Holocaust survivors’ faces either cut out or torn in two.

The attack comes a week after “Against Forgetting” — created by Luigi Toscano in conjunction with ESRA, Vienna’s counselling service for Holocaust survivors — was tagged with pro-Nazi and antisemitic graffiti.

That graffiti included a swastika and the words, “1 Jesus = 6,000,000 Jews.”

The knife attack marks the third time that the portraits have been defiled since they were installed on Vienna’s Ringstrasse at the end of April.

Austria’s president Alexander Van der Bellen said Monday morning he was “deeply affected and concerned” by the “brutal destruction” of the installation, an act he called “shocking."

“'Never again' cannot become mere empty words,” he intoned. “It is something we have to live out daily.”

With this attack, antisemites in Austria are moving “from word to deed,” Oskar Deutsch, president of Vienna’s Jewish community, warned, adding he was thankful no one was hurt.

Mr Deutsch called on Austrian society to reengage in the “struggle against anti-Semitism in the heart of Europe.”

Police continue to search for those responsible for both the graffiti and knife attacks. Austria’s security services are looking to installing CCTV at the site.

In the meanwhile Nesterval, an alternative artistic group of around 60 members, has set up an around-the-clock watch at the installation in order to prevent another attack.

The vigil is planned to last until Friday.