Voter fatigue on Sunday was the result of the first test of opinion since Serbs chose pro-Western President Boris Tadic in June.
"A little less than 35% have turned out," said Zoran Lucic of the independent monitoring group CESID, of the turnout in the capital Belgrade, which accounts for a quarter of the total electorate.
National turnout was likely to be about the same, he said.
"I think the citizens of Belgrade are a little tired of elections," said outgoing Mayor Radmila Hrustanovic.
Democratic party win
Voters this time were choosing 148 municipal councils and mayors and electing a parliament for the northern province of Vojvodina, recent scene of reported strains between Serbs and the ethnic Hungarian minority.
Closely watched was the race for Belgrade city hall where the mayor is to be directly elected for the first time under new legislation creating a more powerful post to improve the run-down city of 1.8 million.
Nenad Bogdanovic of the pro-Western Democratic party led with 33.1% of the vote ahead of Aleksandar Vucic of the ultra-nationalist Radicals with 29.2%. A runoff will be held in two weeks.
Analysts had predicted the election outcome would mirror results of the June presidential vote, with Democrats and Radicals emerging strongest. Early figures suggested the Democrats were ahead.
The race for third place pits Prime Minister Vojislav Kostunica's Democratic party of Serbia against the newcomer Strength of Serbia party of tycoon Bogoljub Karic.