The Belarusian State Security Committee, still known by its Soviet-era name KGB, said it detained more than 30 contractors from private Russian military firm Wagner outside Minsk, the country's capital.
Belarusian authorities have opened a criminal case against more than 30 Russian private military contractors detained earlier this week.
Belarus state TV on Wednesday broadcast footage of more than 30 suspected Russian private military contractors being detained near Minsk after authorities said they had received information that more than 200 fighters had entered the country to destabilise it ahead of the August 9 election.
Security Council secretary Andrei Ravkov said on Thursday the men were being charged for plotting terrorist acts. Some of the captured men had already confessed to attempting to orchestrate a revolution, Belarus authorities shared.
Minsk's handling of the situation risks worsening already strained relations with traditional ally Russia. Moscow has demanded an explanation from Belarus over the detentions.
Lukashenko's opposition accused of Wagner ties
The detentions are the latest twist in an election campaign that has posed the biggest challenge in years to President Alexander Lukashenko who has ruled the east European country with an iron fist for over a quarter of a century.
Authorities in Belarus suspect the husband of opposition presidential candidate Svetlana Tikhanouskaya may have ties to the detained mercenaries, Belta news agency said on Thursday.
The state-run agency also reported that Belarus had launched a criminal case against Tikhanouskaya's husband Syarhei Tsikhanouski, who is in jail, on suspicion of inciting riots.
After weeks of rare and sustained street protests, the opposition has rallied around Tikhanouskaya.
She told reporters on Thursday she would press on with her campaign.
Authorities on Thursday announced additional security measures for campaign events amid fears among the opposition that Lukashenko might use the alleged plot to intensify a crackdown on rivals.
Lukashenko, 65, has accused opponents of being in cahoots with foreign backers to overthrow him and has jailed two of his main election rivals ahead of the election, which he is expected to win despite growing opposition to his rule.
'Plotting a revolution'
Ravkov told reporters that up to 200 mercenaries were still in Belarus and being hunted by law enforcement agents.
Andrey Dmitriev, a presidential candidate, quoted Ravkov as saying that some of the mercenaries had already confessed to plotting "a revolution".
They were trained in the Russian cities of Pskov and Nevel and were mostly bombers and snipers, Dmitriev told reporters after meeting Ravkov.
"We were told that today there are 170 militants in the country, which we already know, trained in subversion and sniper activities," Dmitriev said. The authorities did not rule out shutting down internet access in the country, he said.
Working for Wagner
State media reported that the detained men worked for Wagner, Russia's best-known private military contractor.
The Kremlin said it was unaware of any illegal activity by the mercenaries.
Kremlin spokesman Dmitry Peskov said Moscow wanted an exhaustive explanation from Belarus about their detention and hoped their rights would be observed in full.
"We don't have information about any illegal activity carried out by them," said Peskov.
"We hope to receive information that will allow us to sort this out".
The Russian state denies it uses mercenaries.
Footage of the men's belongings filmed during their arrest showed Sudanese currency and a Sudanese phone card.
That prompted some experts to suggest the men may have been transiting via Minsk en route to Africa.
Lukashenko's report card
A former collective farm manager, Lukashenko is under pressure from critics over what they say is his irresponsible handling of Covid-19 and the economy and alleged human rights abuses.
He says he has delivered years of economic and political stability and that the state covers many of people's core needs just as it did in the Soviet era. A senior Belarusian security official has said that 14 of the alleged mercenaries had spent time in the Donbass region of eastern Ukraine, where Ukrainian troops have fought Russian-backed fighters in a conflict since 2014.
Ukraine's foreign ministry said Kiev would consider extraditing the suspected Russian mercenaries. Belarus and Ukraine agreed to beef up their border controls in the wake of the detentions.