"The turnout is about 38.5%, or some 2.5 million voters," said Zoran Lucic of the Centre for Free and Fair Elections, an independent observers group which has monitored the polls.

Complete official turnout figures may not be released for several days.

The latest electoral fiasco - the third in 13 months - comes amid the worst political crisis in the country since former nationalist leader Slobodan Milosevic was toppled in a popular uprising three years ago.

Legal experts said a fourth attempt to elect a president cannot now be made until January at the earliest, as the parliament of Serbia was dissolved last week and early legislative elections were called for December 28.