Tens of thousands of supporters President Alexander Lukashenko's top election rival have gathered in the Belarusian capital Minsk for a rally, despite an increasing crackdown on the opposition.
More than 60,000 people demonstrated in the capital, Minsk, according to one human rights group, Viasna.
The rally was organised by Svetlana Tikhanovskaya, who is standing against the long-term president.
Alexander Lukashenko has faced a wave of protests over his bid for a sixth term.
Lukashenko has exerted authoritarian control over Belarus in a style reminiscent of the Soviet era since he took office in 1994.
Belarus ban on election challengers stirs protests
Ms Tikhanovskaya, a political novice, decided to stand for election after her husband was barred from running and jailed earlier this year.
Her supporters gathered in a square in Minsk for what the AFP news agency said was the largest opposition protest in the ex-Soviet country in a decade.
They waved flags and balloons printed with opposition slogans.
"People who want a decent life are not criminals," Ms Tikhanovskaya told the crowd. "They are our people, people who want a peaceful change of power."
"Please don't go against your consciences, don't go against your people," she added.
State authorities earlier accused Ms Tikhanovskaya's husband, Syarhey Tsikhanouski, of collaborating with Russian mercenaries to stoke unrest ahead of the election. The popular opposition blogger faces other charges too.
Ms Tikhanovskaya denied the accusations, which came during an intensifying crackdown on opposition activity.
Earlier on Thursday, the authorities arrested 33 suspected mercenaries whom they accused of plotting "terrorism". Russia said it was seeking clarification over the arrests and denied knowledge of the Wagner paramilitary group which Belarus suspects of involvement.
The secretary of the Belarus Security Council, Andrey Rawkow, said investigators suspected as many as 200 Russian mercenaries had entered the country to "destabilise the situation during the election campaign".
Mr Rawkow said 14 of the detained Russians had fought in Ukraine's Donbas conflict, describing their presence in Belarus as a "very unpleasant situation".
Reports by UN investigators, the US military and journalists have documented Wagner group operations in Ukraine, Syria, Libya, Sudan and the Central African Republic. The Russian government denies any links to the group.
The Belarus state news agency Belta has named all 33 Russian suspects and given their dates of birth.
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