By Andrew Osborn and Andrei Makhovsky

MOSCOW/MINSK (Reuters) – Russia demanded an explanation from Belarus on Thursday, after Minsk arrested a group of alleged Russian mercenaries and said they were suspected of plotting “acts of terrorism” before a presidential election.

Belarusian state television broadcast footage on Wednesday of more than 30 suspected Russian private military contractors being detained near the capital Minsk. Authorities said they had received information that over 200 fighters had entered the country to destabilise it before the Aug. 9 election.

A criminal case was opened on Thursday after Belarus said it suspected “the preparation of terrorist acts”. Some of the captured men had confessed to trying to orchestrate a revolution, and Russia’s ambassador was summoned to explain, officials said.

Belarusian authorities say they believe the husband of opposition presidential candidate Svetlana Tikhanouskaya may have ties to the Russian group and have launched a criminal case against him on suspicion of inciting riots, the Belta news agency reported.

Tensions over the arrests risk worsening already strained relations with traditional ally Russia, which has scaled back its economic support for Belarus after becoming disenchanted with the pace of integration between the two countries.

Moscow denies it uses mercenaries. Kremlin spokesman Dmitry Peskov said Moscow wanted an exhaustive explanation from Belarus about the group detained and hoped their rights would be observed fully.

“We don’t have information about any illegal activity carried out by them,” Peskov said in Moscow. “We hope to receive information that will allow us to sort this out”.

Peskov said there were many Belarusian men of a similar description inside Russia, but that Moscow did not assume they were doing anything illegal.

The election campaign has posed the biggest challenge in years to Belarusian President Alexander Lukashenko, who has allowed little dissent in over a quarter of a century in power in the country of 9.5 million between Russia and European Union member state Poland.

Authorities announced additional security measures for campaign events on Thursday. The opposition feared Lukashenko would use the alleged plot to intensify a crackdown on rivals.

Lukashenko, 65, has accused opponents of working with foreign backers to overthrow him and has jailed two of his main election rivals.

He is expected to win the election despite growing opposition and protests over concerns about the economy, human rights and his handling of the COVID-19 pandemic. He portrays himself as a guarantor of economic and political stability.

‘PLOTTING A REVOLUTION’

Belarusian Security Council State Secretary Andrei Ravkov told reporters up to 200 mercenaries were being hunted by law enforcement agents.

Andrey Dmitriev, a presidential candidate, quoted Ravkov as saying some of the people detained had confessed to plotting “a revolution”.

They were alleged to have trained in the Russian cities of Pskov and Nevel and were mostly bombers and snipers, Dmitriev told reporters after meeting Ravkov. The authorities did not rule out shutting down internet access in Belarus, he said.

State media have suggested the detained men worked for Wagner, a private military contractor.

Footage of the group’s arrest showed they had Sudanese currency and a Sudanese phone card with them, prompting some Russian military experts to suggest they were going through Minsk en route to Africa.

(Reporting by Andrei Makhovsky in Minsk, Maxim Rodionov and Tom Balmforth in Moscow; writing by Matthias Williams/Andrew Osborn,; Editing by Timothy Heritage)