The plaque was dedicated by the Croatian government.
Addressing reporters, Kosor said that Stara Gradiska could serve as a symbol of what totalitarian regimes could do to human beings and their dignity.
Lessons can be learnt here, Kosor said, adding that what happened in that jail must not sink into oblivion.
"We must remember all victims equally," she said.
This is the right place to pay tribute to the victims of totalitarian regimes in Croatia, wishing that nothing should be forgotten and that investigations should go on, she said.
We especially pay tribute to the victims of Nazism and the Ustasha regime in Croatia. However, we are now also trying to pave the way for investigations into Communist crimes and to cease treating that issue as a taboo, Kosor said urging open discussions on those crimes.
The European Day of Remembrance for Victims of Stalinism and Nazism is also known as the International Black Ribbon Day. It was designated by the European Parliament in 2008/2009 as "a Europe-wide Day of Remembrance for the victims of all totalitarian and authoritarian regimes, to be commemorated with dignity and impartiality" and has been observed annually by the European Union since 2009.
On Tuesday, this remembrance day was observed in Djakovo, Dubrovnik, Macelj and some other places in Croatia.
The Croatian association of former political prisoners, which goes by the acronym HDPZ, has reported that some 30 graves with Communist victims have so far been unearthed in the area of the eastern town of Djakovo.