By Matt Robinson
BELGRADE (Reuters) - The leader of Serbia's rightist opposition said on Saturday he may pull out of a presidential run-off this month after accusing incumbent President Boris Tadic of rigging the first stage of the election.
Tomislav Nikolic said his Serbian Progressive Party would begin "peaceful, democratic and non-violent protests across Serbia" from Sunday, accusing the dominant Democratic Party of fraud during the presidential and parliamentary vote on May 6.
"A decision ... on whether there will be a second round will be taken tomorrow," he told reporters. "I'm tempted to believe that Boris Tadic didn't even get into the second round."
The second round on May 20 would see Nikolic go head-to-head with Democratic Party leader and pro-Western Tadic, with the incumbent the favorite to win a new five-year term.
Tadic beat Nikolic in the first-round of the presidential race by less than one percentage point. Nikolic's party narrowly won the parliamentary election, but faces being locked out of government by a renewed alliance of the second-placed Democrats and the third-placed Socialist Party.
Nikolic said he did not recognize the official results of the May 6 vote announced by the state electoral commission.
The commission on Saturday rejected allegations made by Nikolic that the Democratic Party defrauded 500,000 votes. His party has produced thousands of what it says are fraudulent ballot papers in sacks.
The Organisation for Security and Cooperation in Europe (OSCE), which monitored May 6 elections, said they were largely free and fair but noted "some lack of transparency" in how the voter register was compiled.
Electoral commission president Predrag Grgic said there were no grounds for annulling the elections.
"All complaints that the republic election commission has received so far have been rejected as late or unfounded and there is no reason whatsoever for the elections to be annulled," he said in a statement.
The Democratic Party, Serbia's dominant political force since strongman Slobodan Milosevic was ousted in 2000, accused Nikolic on Friday of trying to whip up unrest.
The Democrat-Socialist coalition, in power for the past four years, has a patchy record on reform but would keep the former Yugoslav republic edging towards membership of the European Union.
(Reporting by Matt Robinson; Editing by Sophie Hares)