Thousands of far-right supporters have gathered on a field in southern Austria to commemorate the massacre of Croatian pro-Nazis by victorious communists at the end of World War II.

The event on Saturday, which featured insignia and flags of Croatia's wartime pro-fascist Ustasha regime, comes amid a surge of far-right sentiments in the EU's newest member country.

Tens of thousands of Croatians, mostly Ustasha soldiers, fled to Bleiburg in May 1945 amid a Yugoslav communist offensive, only to be turned back from Austria by the British military and into the hands of revengeful antifascists. Thousands were killed and buried in mass graves in and around Bleiburg.

For Croatian nationalists, the Bleiburg site symbolizes their suffering under communism in Yugoslavia before they gained independence in the 1990s.