As demonstrators began erecting small camping tents, police moved in and confiscated one. No violence was reported.
Protesters are chanting "Freedom," "Long Live Belarus!" "We Shall Win!" and Milinkevich’s name. Others are holding lit candles and waving flags adorned with the traditional red and white colors of Belarus.
A pro-opposition Belarusian website -- Khartiya '97 -- says RFE/RL's Belarus Service's programs are being broadcast live on the square.
The crowd voted a "declaration of the Belarusian people" that accuses incumbent President Alyaksandr Lukashenka of falsifying the vote.
The Central Election Commission says Lukashenka won the election with nearly 83 percent of the votes. Milinkevich came second with an official six percent.
International observers have described the election as falling behind democratic standards.
Election Day In Images
THE BELARUS ELECTION: To view a complete archive of RFE/RL's coverage of the March 19 presidential election, click here.
To read and listen to RFE/RL's Belarus Service's coverage of the vote in Belarusian, click here.