"This is an anti-constitutional coup," Alexander Milinkevich told an opposition rally in central Minsk.
Milinkevich, who won 4.4% of the vote, said the opposition would never accept the results.
An opposition rally is currently under way in the Belarusian capital. Police said about 5,000 people have gathered in the city's central square. The atmosphere at the rally is calm.
The rally follows incumbent President Alexander Lukashenko's landslide victory in Sunday's elections.
Lukashenko, dubbed "the last dictator in Europe" by Washington, won more than 80% of the vote to sweep to a victory that had been widely predicted by both his supporters and the opposition.
Despite criticism in the West, Lukashenko is known to have support in his homeland for providing relative economic stability in comparison with other former Soviet states.
