Tomislav Nikolic, a leader among those nationalists and a presidential candidate, promised to provide more help for ordinary people and to fight corruption in Serbia's government, which is now led by pro-European Union reformists.
"Vote for us, bring a new hope to Serbia," he told the cheering crowd at a central square in the capital. "That's the only way to bring about change."
In the May 6 vote, Nikolic and his Serbian Progressive Party are challenging the governing Democrats, led by Boris Tadic. Tadic's government has led Serbia toward European Union membership, following an era of wars and international sanctions in the 1990s.
The Balkan country formally became a candidate for EU membership in March, but its economy remains week amid high unemployment.
Nikolic and his party were once allied with late autocrat Slobodan Milosevic, but recently have shifted to a more moderate position. They have accused Tadic of poor economic policies, pledging to further strengthen economic and political ties with Russia, if they win the vote.
The two camps are close to each other in the polls. However, analysts said the ballot could leave no party with a legislative majority and are predicting a presidential runoff.