Addressing the event, party chief Josip Friscic presented a joint election programme to be discussed with the coalition partner, the Croatian Social Liberal Party (HSLS).
Friscic also announced a possibility for some regional parties to join the HSS-HSLS coalition before next year's elections.
He dismissed objections that the HSS was a Euro-sceptic party because it insisted on the immediate enforcement of the Ecological and Fisheries Protection Zone to cover Italy and Slovenia.
"The HSS is not a party of Euro-sceptics, but we have always warned that the struggle for national interests is our right," Friscic said.
In attendance were guests from the Social Democrats (SDP), the Croatian People's Party (HNS) and HSLS representatives, as well as an envoy from the Office of President Stjepan Mesic, and the head of the Federation of Independent Croatian Trade Unions (NHS), Kresimir Sever.