About 1,500 people have gathered on the square to support the Belarusian opposition in their protest against incumbent leader Alexander Lukashenko's March 19 landslide triumph.
Opposition representatives said Sunday's election, in which the former collective farm boss was reelected to a third five-year term with 82.6% of the vote, was rigged and demanded a re-run.
Washington has dubbed the president "Europe's last dictator" for his alleged authoritarianism, though he is known to have support in his homeland for providing relative economic stability in comparison with other former Soviet states.
The protesters have put up tents in the square and are planning to stay there all night, despite forecasts that the temperature in the Belarusian capital will fall to minus 8 °C (17.6 ° F).