Authorities in Belarus say they have arrested 33 suspected Russian mercenaries believed to be plotting "terrorism" ahead of the country's 9 August presidential election.
The suspects are alleged to be part of a shadowy military group.
Russia is seeking clarification on the arrests, Kremlin spokesman Dmitry Peskov said, denying any knowledge.
President Alexander Lukashenko has been in power since 1994 and is seeking a sixth term in office.
He has exerted authoritarian control over Belarus in a style reminiscent of the Soviet era and has faced a wave of opposition protests over his re-election bid.
Since the collapse of the USSR in 1991 Russia has maintained close ties with Belarus and they have held joint military exercises.
The secretary of the Belarus Security Council, Andrei Ravkov, said investigators suspected as many as 200 Russian mercenaries had entered the country to "destabilise the situation during the election campaign".
The alleged mercenaries arrested on Thursday are believed to be part of the Wagner PMC (Private Military Company).
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.
The Investigative Committee (SK), handling the case, says it is examining possible links between the Russian suspects and the jailed husband of opposition presidential candidate, Svetlana Tikhanovskaya.
Sergei Tikhanovsky, a popular opposition blogger, is accused of trying to foment mass unrest, despite being in jail. He faces other charges too.
"Tikhanovsky, [Mikola] Statkevich and 33 detained Russian citizens - they acted together," SK spokesman Sergei Kabakovich told the AFP news agency.
Mr Statkevich, a veteran opposition activist, is also in jail.
Andrey Ravkov said 14 of the detained Russians had fought in Ukraine's Donbas conflict, describing their presence in Belarus as a "very unpleasant situation".
Meanwhile, the Ukrainian SBU security service says it plans to seek the Russians' extradition, so that they can be prosecuted for alleged crimes in eastern Ukraine.
Russia is under international sanctions for its military involvement in Ukraine, though the Kremlin denies arming the pro-Russian separatists in Donbas.
The state TV channel Belarus 1 showed the Russians being arrested by the Belarus KGB at a sanatorium. The video report (in Russian) said the group had raised suspicions because of their disciplined, military-style behaviour - for example they avoided alcohol.
Read more about the Wagner group:
The SK statement said that on Wednesday the authorities had "arrested 33 citizens of the Russian Federation, members of the Wagner PMC [private military company], who were staying at the Belorusochka sanatorium in Minsk district".
Video footage of the group's arrest showed they had Sudanese currency and a Sudanese phone card with them. The Wagner group is known to be active in Sudan, and there is speculation that it was using Belarus as a transit country for African operations.
In Moscow, Mr Peskov said on Thursday: "We know 33 Russians were detained yesterday in Belarus. Two hundred are still at large. We do not know of any illegal activity that would be grounds for their detention."
He also said there was "no such concept" as "Wagner PMC" in Russia.
"There are already insinuations that some organisations from Russia are sending someone to destabilise the situation in Belarus. Or course it is nothing but insinuations," he said.