Serbs in Bosnia headed to the polls on Sunday to decide whether to mark their "national holiday", in a referendum that has placed the country's fragile institutions under pressure.

The vote is the brainchild of Milorad Dodik, nationalist leader of the Bosnian Serb-run entity Republika Srpska (RS).

He has ignored a veto by Bosnia's constitutional court, disapproval by the United States and the European Union and the reservations of Serbia, RS' big ally.

The referendum, on whether to mark Jan. 9 as "Statehood Day" in the Serb Republic part of Bosnia, will be the first since a 1992 plebiscite on secession from then-Yugoslavia that ignited three years of ethnic war in which 100,000 were killed.

Some 1.2 million voters are entitled to cast a ballot on whether they want their "national holiday" to continue to be celebrated on January 9. Polls opened at 7:00 am local time (0500 GMT) and are to close at 7:00 pm (1700 GMT).

"I came to vote because every nation and every state has its own national holiday. Accordingly, our Serbian people must have their holiday," said Vojo Vujakovic, 60, at a polling station near the Bosnian Serb capitol of Banja Luka.

The date has huge emotional resonance in Bosnia, stirring memories of nationalist fervor, trauma and bloodshed.

It marks the proclamation of a "Republic of Serb people" in Bosnia that took place three months before the inter-ethnic 1992-1995 war that claimed 100,000 lives.

The founders of that "republic" included Radovan Karadzic -- sentenced in March to 40 years' jail for genocide and crimes against humanity committed during the war that also displaced more than two million people.

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, but Dodik pressed ahead defiantly.

'Playing with fire'

"The Republic is going into a referendum. It's a great day for our Republic and our people," Dodik said on Friday after a trip to Moscow, where he met Russian President Vladimir Putin.

"We have to show our dignity, that we are a democratic people and that we have the right to make our own decisions," he said.

The Dayton peace agreement that ended Bosnia's war split the country into two semi-independent entities -- the RS and a Muslim-Croat Federation.

But analysts say that the institutional bonds between the entities remain weak and prone to instability.

Some fear Dodik is warming up for a referendum on the independence of RS, which he has repeatedly threatened to carry out.

Bosnian Muslim leader Bakir Izetbegovic has accused Dodik of "playing with fire", and a wartime commander of Bosnian Muslim forces, Sefer Halilovic, has accused him of "crossing the red line."

But some commentators say a conflict is unlikely and a crisis is being stoked to boost nationalists' chances in upcoming local elections.

Anecdotal evidence points to strong support from the public for the vote.

"We are all going to take part, to preserve our freedom, peace and dignity," Vida Kojic, a pensioner in Lukavica, a suburb of Sarajevo that lies in the RS, told AFP.

Many roadsides in the RS had placards urging people to turn out and cast their ballot, and some signs portrayed Dodik along with the message, "The force of Srpska."

"This day is important for me to confirm why I took part in the war and lost my arm," said Novak Kajkut, 45, a war invalid, while waiting to cast his vote in the region's capital of Banja Luka. "We don't dispute the right of the Muslim Bosniaks to mark their holidays but they can neither dispute this right to us."

The Serbs celebrate the holiday by hanging out Serb flags and holding Orthodox Christian ceremonies in public institutions, which non-Serbs say is aimed at excluding them.

Many believe that by defying the court ruling, Bosnian Serb leader Milorad Dodok is aiming to highlight the weakness of post-war Bosnia's central authorities in Sarajevo and set the stage for a vote on secession.

Western diplomats warn that the referendum violates the 1995 Dayton peace accords that ended the Bosnian war.

Some Bosniak politicians said it would weaken Bosnia's delicate structure, created to hold the country together in the aftermath of the devastating war. Many in the Bosniak- and Croat- dominated autonomous region fear that the Serb Republic could be preparing to secede, bringing the future of Bosnia as a whole into doubt.

Talk of a new war has increased tensions, prompting the Serb Republic police to raise the security level at the weekend.

"What's happening brings back memories of what happened in 1992," when the Bosnian war began, said Nusreta Sivac, a Bosniak who was held in a Serb detention camp in the western town of Prijedor during the war.

While the United States, which brokered the Dayton treaty, and the European Union called on the Serb Republic to cancel the vote, fearing instability, Russia supported the plebiscite.

"The West and Russia are choosing sides again - whenever big powers get involved, people suffer," Bosnian Serb opposition leader Mladen Bosic told Reuters.