By Aleksandar Vasovic
BELGRADE (Reuters) - Serbian opposition leader Tomislav Nikolic threatened on Friday to challenge the result of a presidential run-off on May 20, with reformist incumbent Boris Tadic ahead in a new opinion poll.
Accusations of fraud, lodged by the rightist opposition following presidential and parliamentary elections on May 6, threaten to mar the second-round presidential vote.
Nikolic, 60, said on Friday he was considering calling supporters into the streets ahead of the election. Tadic, president since 2004, expects to win another five years in power.
Nikolic's Serbian Progressive Party narrowly won the parliamentary election, but faces being locked out of government by a renewed coalition of Tadic's second-placed Democratic Party and the third-placed Socialists of late Serb strongman Slobodan Milosevic.
The alliance has a patchy record of reform over the past four years but would likely keep the former Yugoslav republic edging towards the European Union.
Nikolic, a former ally of Milosevic who has tried to shed his ultranationalist image, accused the Democrats on Thursday of stuffing the voter register with the names of dead Serbs and stealing 500,000 votes.
His party filed a criminal complaint on Friday.
"I won't rest until this is investigated and I'm seriously thinking about what we'll do in the elections on May 20," Nikolic told an election rally near the southern town of Krusevac, the Beta news agency reported.
"I'm seriously thinking about calling people to Belgrade a day before the elections, to show that band that we won't put up with it, that we're more proud, honest, respectable, stronger and braver than they think we are."
The state Election Commission has dismissed the accusations as unfounded. Monitors from the Organisation for Security and Cooperation in Europe (OSCE) said the elections were largely free and fair but noted "some lack of transparency" in how the voter register was compiled.
Democratic Party deputy leader Jelena Trivan said Nikolic had "returned to his radical rhetoric and behavior".
"Nikolic today openly threatened violence and unrest on the streets and I call on all state institutions to treat this information with the required seriousness," Trivan told the state news agency Tanjug.
The Democrats and Socialists expect to secure the support of another junior partner for a majority of 126 in the 250-seat parliament after the presidential run-off.
Belgrade-based pollster Ipsos Strategic Marketing said Tadic would take 58 percent of the vote over Nikolic on 42 percent, based on a poll of 1,198 voters. The poll has a three percent margin of error.
Tadic beat Nikolic in the first round by less than one percentage point, but endorsements since then from political allies are likely to sway the electorate in Tadic's favor.
(Writing by Matt Robinson; editing by Andrew Roche)