Polls opened on Sunday (September 25) in a referendum in the Serb Republic part of Bosnia over a disputed national holiday.
The referendum, whether to mark January 9 as "Statehood Day", is being held despite being ruled unconstitutional by the state Constitutional Court.
It has also been opposed by the European Union and United States over fears it could risk stoking ethnic tensions in the divided Balkan country.
The referendum will be the first since a 1992 plebiscite on secession from then-Yugoslavia that ignited three years of ethnic war in which 100,000 people were killed.
January 9 is the date when Bosnian Serbs declared independence from Bosnia in 1992, precipitating the country's devastating war marked by mass killings and persecution of Bosniaks and Croats in the territory they earmarked to become exclusively Serb.
The Sarajevo-based Constitutional Court has ruled that the holiday would be illegal because it coincides with a Serbian Orthodox Christian holiday and so discriminates against Muslim Bosniaks and Catholic Croats living in the Serb Republic.
The court also banned the referendum.
