Speaking to journalists in Moscow, Alyaksandr Milinkevich said President Alyaksandr Lukashenka's determination to stay in power and widespread doubts about the legitimacy of the 19 March elections could make for a volatile mix.
He urged Russia to drop its policy of support for the Belarusian leader.
Most of the Belarus opposition has united around Milinkevich as its main candidate to challenge Lukashenka, who is standing for a third term.
On 15 February, Austrian State Secretary for Foreign Affairs Hans Winkler said he thought Belarus's election would not bring about "sweeping change." Austria holds the current EU presidency.