Updated

Serbian nationalists accused pro-European Union reformists Thursday of stealing the recent general elections, fueling tensions ahead of a key presidential runoff.

Progressive Party leader Tomislav Nikolic alleged that the Democratic Party printed extra ballots and tampered with voting lists in Sunday's vote. He presented a sack stuffed with thousands of voting ballots that he claimed were replaced by other ballots by the pro-EU camp and alleged "hundreds of thousands of ballots" were switched in such a way.

"This was an election robbery of an unprecedented scale," Nikolic said during a hastily called press conference.

Democratic Party leader Boris Tadic, who will face Nikolic in the May 20 runoff, has rejected the allegations, saying they were designed to undermine his presidential bid. Tadic described the elections as "a normal one with sporadic mistakes."

Tadic was half a percent ahead of Nikolic in the presidential first round, while the Progressives led the parliamentary vote. The result of the runoff could affect Serbia's bid to join the EU, a bid led by Tadic. Serbia formally became a candidate for entry in March.

Dveri, a nationalist group, was planning a rally to protest the election fraud but said Thursday that police had banned it.

Election observers have said that Sunday's vote was free and fair.

Marko Blagojevic, from the respected Center for Free Elections and Democracy, said "it would be quite odd to miss an election robbery of such proportions" as alleged by the nationalists.

The third-place Socialists have said they will form a new government with Tadic's Democrats.