Further delay in his extradition to the UN war crimes tribunal, where he faces 11 charges including genocide, could undermine President Tadic's pro-Europe government.
Despite Russian and Serbian opposition, UN Secretary-General Ban Ki Moon said Friday the UN would gradually cede its role.
On Sunday, Serbs will weigh in on whether to pursue a pro-European Union path or a more nationalistic, anti-Western route.
Russia is sending humanitarian supplies worth $1.7 million directly to Kosovo Serbs, challenging the authority of the US-backed government in Pristina.
The clash between UN peacekeepers and Serb protesters at a Mitrovica courthouse could bolster hard-liners in Belgrade ahead of May elections.
He won the presidency, but Radical Party's Nikolic will probably still be influential.
Political motives lay behind the decision of Serbia's second-largest city to declare Vladimir Putin an 'honorary citizen'
The Radical Party acting leader Nikolic got 39 percent of the vote in presidential polls Sunday. He and pro-European moderate Tadic will square off in Round 2 of voting on Feb. 3.
Presidential polls take place Sunday amid intense debate that has pitted hard-line nationalists against more moderate politicians.