Polling stations across Croatia opened early, at 5 a.m. local time, and closed at 5 p.m. with initial results expected later in the evening.

Surveys indicate that some 60 percent of Croatians back entry into the European Union. A "yes" vote would pave the way for the former Yugoslav republic to formally join the bloc in 2013 if its accession is approved by all 27 current EU members.

The importance of EU membership is one of the few issues on which all major Croatian political parties agree. Croatia's leaders have long said that joining the EU has been a strategic goal since Zagreb won its independence following the 1991-95 war with rebel Serbs.

President Ivo Josipovic has called the vote "one of the most important decisions in Croatia's history."

The referendum requires a simple majority regardless of the turnout.

Of the six former Yugoslav republics, only Slovenia is an EU member, although Serbia, Montenegro, Macedonia and Bosnia all also have aspirations.

Convicted war criminal urges 'yes' vote

In the 1990s, when other post-communist countries in Eastern Europe were strengthening their democracies and moving toward EU integration, Croatia's EU aspirations were halted by the 1991-95 war.

It was not until 2000 that the election of a pro-European leader enabled Croatia to pursue a genuine parliamentary democracy making it eligible for EU candidate status.

However, many of the criteria imposed by Brussels, notably full cooperation with the UN war crimes court, were seen by many as a form of blackmail and contrary to national interests.

In a surprise move on Saturday, former General Ante Gotovina, whose flight from the UN tribunal hampered Croatia's bid, urged voters to cast a "yes" vote. Gotovina, still viewed by some as a national hero, is being held in prison at the Hague-based court which sentenced him in 2011 to 24 years in jail on war crimes charges. His conviction sparked a surge of anti-European sentiment in Croatia.

Those who support EU membership say the Balkan country's troubled economy would benefit from better access to markets, but opponents argue that nothing would be gained by joining and that it would only lead to a loss of sovereignty and national identity.

Author: Gregg Benzow (AP, AFP, Reuters)

Editor: Sean Sinico