Serbia has failed to elect a president for the third time in just over a year because of low turnout, exit polls suggest.
Micunovic appealed for a "stable" Serbia
Independent observers say just over 38% of the electorate voted, well short of the 50% needed to make the poll valid.
Unofficial results indicate nationalist Tomislav Nikolic leads the field, in a vote boycotted by the main opposition.
Government candidate Dragoljub Micunovic appears to have been pushed into second.
Correspondents say the outcome is a setback for the ruling pro-Western coalition as it seeks a mandate for reform at December's parliamentary elections.
Two main opposition parties are claiming the election is irrelevant, while many people have become disillusioned after a spate of well publicised scandals involving members of the government, our correspondent says.
To make it even harder to achieve the necessary turnout, it is believed half a million potential voters live abroad and could have only cast their ballots if they returned home.
With a general election having just been announced for 28 December, many people may be waiting until then before giving their verdict on the country's politicians.
If one of the candidates is elected he will be the first democratically elected president of Serbia since the fall of Slobodan Milosevic's regime in what was then Yugoslavia in October 2000.
The reformist government of Prime Minister Zoran Zivkovic has been struggling since the assassination of former premier Zoran Djindjic by a sniper in Belgrade on 12 March.