RFE/RL's Belarus Service reports that up to 6,000 people have already gathered on Kastrychnitskaya Square in the center of the capital, chanting "Freedom" and "Long live Belarus."
Reports say busloads of riot police have already streamed into the area and that more troops are arriving.
An estimated 10,000 opposition supporters rallied in Minsk on March 19 after the vote. Authorities did not prevent the demonstration, which ended with no incident.
Belarus's main opposition candidate, Alyaksandr Milinkevich, today accused incumbent President Alyaksandr Lukashenka of seizing power through unconstitutional means.
The Central Election Commission said Lukashenka won the election with nearly 83 percent of the vote. Milinkevich finished second with an official 6 percent.
International observers have described the election as failing to meet democratic standards.
(RFE/RL's Belarus Service)