Fresh strikes are expected in Belarus after a weekend in which tens of thousands took to the streets to demand the departure of longtime President Alexander Lukashenko.
Opposition leaders called for a boycott of the rally on Friday, removing hundreds of protesters by truck.
But the president, who claimed a landslide victory, remains defiant.
Sunday, he called on supporters to defend their country and independence.
Protestants, however, came out in much larger numbers, with local, independent news website Tut.by describing the peaceful demonstration as “the largest in the history of independent Belarus”.
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The wave of anger has increased since the Central Election Commission said Mr Lukashenko, who has been in power since 1994, won 80.1% of the vote and main opposition candidate Svetlana Tikhanovskaya 10.12%.
Ms Tikhanovskaya notes that, where votes were well counted, she won support from 60% to 70%. She went to the country after announcing the results publicly, and joined her children in Lithuania.
Meanwhile, about 6,700 people have been arrested in the wake of the election, and many have spoken of torture at the hands of the security services.
Employers at state-owned factories ran out of solidarity with the Protestants last week, and more strikes are planned for this week, increasing pressure on Mr Lukashenko, says BBC’s Kiev correspondent Jonah Fisher.
What happened on Sunday?
Rival rallies were held in the capital, with local media reports suggesting that about 31,000 people attended the pro-government event on Sunday. The Interior Ministry estimated the number at around 65,000, while a reporter for the AFP news agency said the figure was closer to 10,000.
Speaking with supporters, Mr Lukashenko said Belarus would “die as a state” if a re-run of the elections took place.
“You came here so that for the first time in four years you could defend your country, your independence, your wives, sisters and children,” he said.
He added that the opposition “would like rats out of a hole” if they were not suppressed this time.
There were reports of state workers being forced to attend or face the threat of losing their jobs. For days, workers at state-run factories staged walkouts and many took part in street marches against the president.
While the president is speaking, some 220,000 anti-Lukashenko protesters are gathering near the Stela Minsk Hero City World War Two memorial in central Minsk, according to the news website Tut.by.
Supporters also turned out in other cities, following a call for weekend rally from Ms Tikhanovskaya.
A number of officials, such as current and former police officers, have been fired.
The Belarusian ambassador to Slovakia, Igor Leshchenya, declared his solidarity with the Protestants.
He told the BBC, however, that he had “gloomy prophecies” about the government’s willingness to listen to Protestants.
“It’s not obvious that government is ready to hear them,” he said.
What is happening internationally?
President Lukashenko, who has led Belarus for 26 years, is also facing growing pressure from other European countries.
On Sunday, French President Emmanuel Macron said the EU “must continue to mobilize on the side of the hundreds of thousands of Belarusians who are peacefully protesting for respect for their rights, freedoms and sovereignty”.
More on the protests in Belarus
German Chancellor Olaf Sholz, meanwhile, described Mr Lukashenko as a “bad dictator” and told the Bild newspaper that the Belarussian leader had “lost all legitimacy”.
Last week, EU foreign ministers agreed to prepare new sanctions against Belarusian officials responsible for “violence, repression and the falsification of election results”. The US has condemned the election as “not free and fair”.
Hundreds of people attended protests in Prague and Warsaw on Sunday.
But President Lukashenko has sought Russian help as the unrest continues.
On Saturday, he said that President Vladimir Putin had promised what he called comprehensive assistance in case of external military threats to Belarus.
The two leaders held a second meeting on Sunday, in which the Kremlin said it was discussing “the situation in Belarus, taking into account the pressure exerted on the republic from outside”.
Mr Putin told Mr Lukashenko Russia was ready to help Belarus “in accordance with the collective military pact as needed”.
The longtime Belarusian leader also worried about the NATO military exercises taking place in neighboring Poland and Lithuania and launching a tirade against the Western military alliance.