"Long live Belarus! Long live Belarus! Long live Belarus!..." they shouted.
RFE/RL's Belarus Service reported from the scene that between 20,000 and 30,000 protesters have flooded onto the city's October Square, despite the closure of metro and bus service in the area.
Opposition candidates Alyaksandr Milinkevich and Alyaksandr Kazulin had both come to the square to address the crowds.
"The square has been filled with people for three hours now," reported RFE/RL correspondent Alexander Lukashuk from the square. "The last time Minsk saw such a large number of people and such a large number of white-red-and-white flags was when it said good-bye to [Belarusian writer and activist] Vasil Bykau."
"[There are] an incredible number of people, an incredible number of young people," Lukashuk reported. "There are very young and beautiful faces here. And of course there are a lot of white-red-and-white flags, yellow-and-blue [Ukrainian] flags, European Union flags and flags of the Zubr movement."
The head of the Belarusian Central Election Commission, Lidziya Yarmoshyna, said preliminary election results will be announced in the next several hours.
But she said early figures from "closed" polling stations like hospitals showed incumbent Alyaksandr Lukashenka with an overwhelming lead.
"The incumbent president's victory is even more overwhelming [in military units]," Yarmoshyna said. "He won 98.4 percent of the vote there."
Lukashenka, who has ruled Belarus for 12 years, is seeking an unprecedented third term that would keep him in office until 2011.
(with material from agency reports)
Alyaksandr Milinkevich attending an opposition rock concert on March18 in Minsk, where he first issued a call for protests (RFE/RL)
TOWARD A SHOWDOWN?: The main Belarusian opposition presidential candidate, Alyaksandr Milinkevich, and his campaign manager, Syarhey Kalyakin, spoke on March 19 at a news conference in Minsk. RFE/RL's Belarus Service provided excerpts from Milinkevich's remarks.
Milinkevich: I believe this civil confrontation can be resolved by repeating elections, in which representatives of all candidates will be represented in election commissions.
Milinkevich: This will not be a true result. It will be a result of falsifications that have been continuing for a long time. [The results] will be recognized neither by us nor the majority of the Belarusian population. They will not be recognized by democratic countries, it has already become obvious.
Milinkevich: I will be appealing to the good senses of the authorities. This conflict should not be escalated. It should not be fomented. A resolution should be found. Let's sit down to the negotiating table.
Milinkevich: I am absolutely confident that democratic countries will not recognize this election as legitimate. I very much hope that there will be quite a few serious politicians in Russia who will do the same.
Milinkevich: Those people who will overcome fear will come out in the streets. There was and still is fear being infused today by warnings of alleged terrorism, which has never been heard of in Belarus. So let's see how many people will come out. I will be grateful to everyone who comes out and says peacefully that we want to live like humans and not on our knees, in humiliation.