Montenegro voted Sunday on whether to become an independent state -- and write the final chapter in the breakup of former Yugoslavia -- or stay in its troubled union with Serbia.

The referendum, which will decide if Europe will add another tiny state to its map, has caused tension between those wanting to remain in Serbia-Montenegro and those seeking to restore Montenegro's 1878-1918 statehood.

The division between anti- and pro-independence groups is deeply rooted in the history of the small Adriatic Sea republic. There are fears that whichever way the vote goes, there could be violence after the results are announced.

People lined up in front of polling stations as they opened in 21 municipalities across Montenegro at 8 a.m. (0600 GMT). The stations close at 9 p.m. (1900 GMT), and first official results are expected Monday.

The latest opinion polls indicate that a slight majority of Montenegrins will vote for independence.

But it is not clear whether the pro-independence camp will collect at least 55 percent of "Yes" votes, set as a rule by the European Union for the vote to be valid.

The Serbia-Montenegro union is the last shred of the federation of Yugoslavia that began its blood-drenched breakup in the early 1990s. Now, Montenegro's 485,000 voters must decide whether to break away from Serbia -- their partner republic that is eight times their size.

Once an independent kingdom, Montenegro was erased from the map after World War I and merged into the newly formed Yugoslavia. Many Montenegrins resisted and a seven-year guerrilla war followed. After World War II, the six-republic Yugoslavia became communist.

During the federation's violent breakup in the 1990s, Montenegro's leaders sided with Serbian President Slobodan Milosevic -- who would later stand trial for war crimes -- in his war campaigns in Slovenia, Croatia and Bosnia.

But relations soured, and the EU -- fearing further fragmentation of Balkan states -- in 2002 brokered a deal to keep Serbia and Montenegro together.

Now, Montenegro's Prime Minister Milo Djukanovic, a former Milosevic ally, supports ending the union. Serbia's nationalist Premier Vojislav Kostunica disagrees.

Djukanovic argues that this impoverished but spectacular country of soaring mountains and stunning Adriatic coastline is being stifled by Serbia.

His independence camp says breaking away will boost the economy and speed the country's path to joining states like Slovenia, also a former Yugoslav republic, in the prosperous European Union.

The pro-Serbian camp says Montenegro, with a population of 620,000, is too small to be viable on its own.

The EU, fearing violence, has set the 55 percent validity threshold in an effort to avoid an unconvincing majority for such a crucial decision. The EU brokered the referendum laws after Montenegro's two rival factions failed to agree on the rules for the vote.