Serbs were told to display thousands of red, blue and white national flags wherever they live in the region or the world to mark "The Day of Serb Unity, Freedom and the National Flag."

Opening the full day of celebrations, Serbia's populist President Aleksandar Vucic inspected military hardware displayed in a Belgrade park, praising the army's readiness to respond to outside threats.

He said that the army is "five times stronger" than only a few years ago and announced new military purchases.

The flexing of muscles by Serbian officials as well as their calls for the creation of the "Serb World," or political unification of an estimated 1.3 million Serbs who live in Bosnia, Montenegro, Kosovo and Croatia with Serbia, have triggered worries in the neighbouring states.

In the 1990s, Serb forces with financial and political support from Belgrade led bloody campaigns in Bosnia, Croatia and Kosovo with the goal of forming a "Greater Serbia."

The campaign unsuccessfully tried to redraw the internal borders of former Yugoslavia and create a single Serb state.

The renewed calls for pan-Serb unity have further raised tensions in the Balkans.

Croatian President Zoran Milanovic said he couldn't "believe that Serbs have nothing more important or smarter to do" than create holidays which infringe on the internal affairs of neighbouring states.

Serbia's Interior Minister Aleksandar Vulin, the most vocal supporter of the "Serb World," was quick to respond.

"There is nothing more important than the preservation of the Serb identity," he said.

The new national holiday coincides with the day when the Royal Serbian army together with French troops defeated Axis forces in a famous World War I battle in northern Greece in 1918.

Australian Associated Press