BELGRADE, Yugoslavia -- Serbia's first presidential elections since Slobodan Milosevic was ousted from power were ruled invalid Sunday because of low turnout, independent vote monitors said.

Widespread voter apathy -- triggered by the slow pace of reforms, quarreling between pro-democracy leaders who ousted Milosevic and low living standards -- produced a turnout of 45.5 percent, according to the independent Center for Free Elections and Democracy.

The legal minimum for valid elections was 50 percent.

"Definitely, there is no possibility that these elections succeeded," said Zoran Lucic, a spokesman for the group. "We have not reached our goal with these elections and we did not elect a president."

Lucic said that exit polls showed that Yugoslav President Vojislav Kostunica won 66.7 percent of the votes, and that deputy Prime Minister Miroljub Labus had 31.3 percent. But with Sunday's vote invalid, Serbs will have to re-launch the election process by Jan. 5, 2003.

Official turnout figures and results were not expected before Monday.

In 1997, a vote for the Serbian presidency failed because of low turnout. A new round of voting the same year led to the election of Milan Milutinovic, the current Serbian president and a former Milosevic ally. Milutinovic -- who kept a low profile after Milosevic's ouster -- is also wanted by the U.N. tribunal for alleged war crimes in Kosovo.

Kostunica on Sunday criticized the Milosevic-era election law that requires large turnouts and two rounds of voting, calling it "irrational."

Kostunica has repeatedly said that a failure to elect the Serbian president would inflict "instability, tensions and chaos" on the republic and jeopardize unfinished reforms.

Kostunica, a moderate nationalist, finished first in the first round of the elections on Sept. 29, but failed to get a majority needed for outright victory. Kostunica succeeded Milosevic as Yugoslavia's president, who was ousted in October 2000.

Turnout was 55 percent in the first round, but Milosevic's allies -- including ultranationalist leader Vojislav Seselj -- who failed to make it to the runoff urged their supporters to boycott the vote.

"It's a pity that these elections did not succeed," said Slobodan Samardzic, an adviser to Kostunica, blaming the outcome on the "outdated" election law that requires a high turnout.

Ultranationalist leader Vojislav Seselj, who failed to advance beyond the Sept. 29 first round of the elections, had urged his supporters to boycott the elections. But many Serbs had other reasons to stay home.

Many in Serbia had hoped for a faster improvement in living standards after Milosevic. Average salaries have gone up, but have barely kept pace with soaring prices despite the relative stability of the national currency, the dinar. Unemployment stands at a staggering 40 percent.

Galvanizing voters into action was a challenge for Kostunica and Labus. Both advocate economic reforms, membership in the European Union and cooperation with the West, but disagree over the best way to achieve those goals.

The most contentious issue between the candidates centered on a power struggle between Kostunica and Labus' powerful backer, Serbia's Prime Minister Zoran Djindjic.

Kostunica and Djindjic have argued almost daily over the pace and style of reforms after they jointly led the rebellion against Milosevic. Kostunica has promised to bring down Djindjic's government if he wins the presidency.

Kostunica will lose his current job when the post of Yugoslav president disappears later this year. Under an EU-supported plan, a new country is to be formed that turns the republics, Serbia and Montenegro, into a loose union.

Only hours before the polls closed, the Serbian Orthodox Church's influential patriarch, Pavle, had issued a dramatic televised appeal for Serbs to vote.

The Patriarch said in a statement that the church was "seriously worried" that a failure of the vote "could cause a significant deterioration of the political situation and upset the functioning of the state and its international relations."