Chanting "Leave!", the protesters marched down Independence Avenue following an opposition call for the biggest rally yet in a week of demonstrations since the vote.
Tens of thousands of Belarusian opposition supporters gathered in Minsk to join a "March for Freedom" over President Alexander Lukashenko's disputed re-election.
Columns of demonstrators raised victory signs and held flowers and balloons. Many wore white, the colour that has come to symbolise the opposition movement. Those marching included a group of veteran paratroopers in uniform berets.
Demonstrators held placards with slogans such as "We are against violence" and "Lukashenko must answer for the torture and dead".
Protesters walked through the city carrying a 100-metre-long red-and-white flag, once the state flag and now used to represent opposition to the current regime.
They chanted "Long live Belarus!" and called for Lukashenko to "Leave!" as cars honked horns in support.
The protests came as Lukashenko held a rally for supporters in central Minsk, urging them to save the country from destruction.
Autocratic Belarusian leader Alexander Lukashenko is under growing pressure with rival protests due to converge on the capital a week after a contested presidential election that has thrown his country into turmoil.
Lukashenko, in power for 26 years, has faced down a week of street demonstrations and refused demands for a re-run of an election protesters say was massively rigged to disguise the fact that he has lost public support.
He denies losing, citing official results that gave him just over 80 percent of the vote.
Often emotional in state TV appearances, the 65-year-old leader has alleged a foreign-backed plot to topple him. He has also cited promised military support from Russian President Vladimir Putin if necessary, something the Kremlin has not confirmed.
Russia, which has had a troubled relationship with Lukashenko, is watching closely as Belarus hosts pipelines that carry Russian energy exports to the West and is also viewed by Moscow as a buffer zone against NATO.
No sign of backing down
The EU is gearing up to impose new sanctions on Belarus in response to a violent crackdown in which at least two protesters have been killed and thousands detained.
Protesters show no signs of backing down.
Metal fencing around Independence Square was being installed on Sunday morning with agricultural vehicles used to close off nearby roads.
Opposition media channels say Lukashenko, a one-time manager of a Soviet-era collective farm, plans to bus people in from other parts of the country and that they will be coerced into attending.
In an unusual move, Igor Leshchenya, the Belarusian ambassador to Slovakia, declared solidarity with protesters in an undated video posted by Nasha Niva media on Saturday. Other state employees, including police officers and state TV staff, have also come out in support of the protests.
Some of the country's biggest state- run industrial plants, the backbone of Lukashenko's Soviet-style economic model, have been hit by protests and walkouts in the past week.
Opposition presidential candidate Tsikhanouskaya, who fled to neighbouring Lithuania on Tuesday, has called for an election recount.
Her campaign has also announced she is starting to form a national council to facilitate a power transfer.
Lukashenko and Putin spoke by phone on Saturday.
Ties between the two traditional allies had been under strain before the election, as Russia scaled back subsidies that propped up Lukashenko's government.
The state news agency Belta on Saturday cited remarks by Lukashenko that "at the first request, Russia will provide comprehensive assistance to ensure the security of Belarus in the event of external military threats".
A Kremlin statement made no mention of such assistance but said both sides expressed confidence that all problems in Belarus would be resolved soon.
Statements by both sides contained a pointed reference to a "union state" between the two countries.
The neighbours signed an agreement in 1999 that was supposed to create a unified state.
That project was never properly implemented, however, and more recently Lukashenko had rejected calls by Moscow for closer economic and political ties as an assault on his country's sovereignty.