The four parties also announced they would form a coalition and participate in the coming parliamentary elections together.
The Serbian parliament last Saturday approved a new constitution, declaring the UN-run province part of Serbia.
The lawmakers -- 242 of them present at the session -- unanimously voted in favor of the constitution, that will replace the current one drafted in 1990 by late former autocratic leader Slobodan Milosevic.
Given that the new constitution needs a yes-vote on a national referendum before becoming valid, the Serbian parliament speaker set the plebiscite for October 28 and 29, urging voters to give the proposed new constitution the legally required two-thirds support and underscore Serbia's opposition to possible independence for Kosovo that might come as the result of the UN-brokered talks on the province.
Kosovo is a province of Serbia but Belgrade has had no authority over the separatist region since a 1999 NATO bombing campaign forced Serb troops to pull out and end a crackdown against ethnic Albanian separatists.