Thousands of supporters of President Alexander Lukashenko's election rival on Thursday rallied in the Belarusian capital Minsk despite an increasing crackdown on the opposition.
Backers of political novice Svetlana Tikhanovskaya, a stay-at-home mother of two, packed a Minsk square in what appeared to be the largest opposition protest in the ex-Soviet country in a decade, an AFP journalist said.
The human rights organisation Vyasna said at least 34,000 people had turned out.
Earlier Thursday, Belarus investigators accused Tikhanovskaya's husband Sergei Tikhanovsky and another prominent Lukashenko critic, Mikola Statkevich, of working together with Russian mercenaries to plot mass unrest ahead of the August 9 election.
Minsk detained 33 Russians on Wednesday.
Belarusian authorities say the men are members of the Wagner group, a shadowy military contractor reportedly controlled by an ally of Russian President Vladimir Putin that promotes Moscow's interests in Syria, Libya and Ukraine.
Addressing her supporters at the rally, Tikhanovskaya, 37, said authorities were "ruining" not only her husband's life but those of all political prisoners.
"The situation involving the fighters is very scary," she said to shouts of "Freedom".
She denied that the opposition was collaborating with the Russians to stage an uprising.
"People, what revolution? We want honest elections," Tikhanovskaya said.
She also said that Russian mercenaries might have been using Belarus as a transit point for a long time.
"If that is so I have a question: where was the security service before and why are they raising this issue right before the election?"
In a statement Thursday evening, the Russian foreign ministry said its nationals were transiting through Belarus because they worked for a Belarus company, adding that they were en route to Istanbul.
Minsk's version of events "does not hold water," the statement said.
"An attempt to make what happened look like foreign interference in the republic's affairs causes bewilderment, to put it mildly," Moscow said.
The foreign ministry urged a halt to fanning tensions ahead of the election.