MINSK, Belarus -- Belarusian authorities accused more than 30 people who allegedly worked for a private Russian military company of plotting terrorist acts and said Thursday that the suspects are also under investigation for “preparations for mass unrest” in connection with an opposition activist jailed earlier this year.

Law enforcement officials in Belarus have opened a criminal case against the Russians, who were detained Wednesday, on charges of plotting terrorist acts, Security Council Secretary Andrei Ravkov said.

The Belarusian State Security Committee, still known by its Soviet-era name KGB, said it detained 32 contractors from private Russian military firm Wagner outside Minsk, the country's capital. Another person was reportedly detained in the south of the country.

Belarus' Investigative Committee said Thursday they were investigating popular opposition blogger Sergei Tikhanovsky, arrested earlier this year, for “preparations for mass unrest,” and that the Russians were detained because of their possible involvement in “said crime.”

Tikhanovsky's wife, Svetlana, is challenging President Alexander Lukashenko in upcoming presidential elections, sparking mass rallies all around the country.

Lukashenko is seeking a sixth term in the Aug. 9 election amid opposition protests fueled by public fatigue with his rule and a painful economic fallout from the coronavirus pandemic. He has repeatedly accused his political opponents of plotting an uprising and alleged that Russian forces interfered with the campaign. Moscow has denied any involvement in its neighbour's election.

Detentions of Russians this week brought political tensions between Russia and Belarus to a new high. The leader of Belarus accused Russia of harbouring “dirty intentions” Wednesday and instructed his officials to ask Russia for an official explanation.

“It's necessary to immediately ask the relevant Russian structures to explain what's going on,” Lukashenko said.

Kremlin spokesman Dmitry Peskov confirmed on Thursday that 33 Russian citizens were detained in Belarus and said that Moscow is hoping to get more information about the incident.

“There is no information about any wrongdoing of the Russians that may have caused the detention. We don't know anything about it,” Peskov said, calling the allegation that Moscow sent private military contractors to destabilize Belarus “nothing but innuendo.”

In Belarus, Ravkov said the search continues for more Russian military contractors. “Up to 200 militants are on our soil,” he said. “It is like a needle in a haystack.”

Wagner - a private military company linked to Yevgeny Prigozhin, a Russian businessman who was indicted in the United States for meddling in the 2016 U.S. presidential election - has allegedly deployed hundreds of military contractors to eastern Ukraine, Syria and Libya.

Peskov said Thursday he was not aware of any private military companies existing in Russia, adding that “there is no such thing as a private military company in Russian law.”

Belarus' Foreign Ministry summoned Russian Ambassador Dmitry Mezentsev on Thursday morning. At the same time, presidential candidates were summoned by the country's election commission. Ravkov informed the candidates about new security measures for campaign events, including a heavier police presence at rallies, and the possibility the government will restrict internet access, some of the meeting attendants told Belarusian media.

Opposition candidate Svetlana Tikhanovskaya said she would not cancel any campaign rallies because of the stricter security. Rallies in her support have been the biggest in years, with the largest one so far drawing about 5,000 people on July 19.

More than 1,000 people have been detained in Belarus since May for taking part in protests, according to the Vyasna human rights centre.

Daria Litvinova in Moscow contributed to this report.