Minsk-based human rights defender Raisa Mikhaylouskaya told RFE/RL that Nyaklyaeu had to call an ambulance as his blood pressure rose as a result of his March 2 argument with the KGB officer.
She said Nyaklyaeu was fine after treatment by paramedics.
Mikhaylouskaya said the incident was kicked off when one of the KGB officers who is permanently present in Nyaklyaeu's apartment refused to leave it when Nyaklyaeu and his wife Volha were about to go to the KGB office in Minsk for questioning.
When the officer refused, Nyaklyaeu protested and refused to report for interrogation.
In the past, when the couple went for questioning, the KGB officers left the apartment for the duration of their absence.
Nyaklyaeu and several other opposition presidential candidates, along with hundreds of their supporters, were arrested on December 19-20 while protesting the results of the presidential election, which they say was rigged.
Nyaklyaeu, 64, was severely beaten during the demonstration and was snatched from his hospital bed the following day and taken to jail.
Incumbent President Alyaksandr Lukashenka was announced the runaway winner of the December vote, which international election monitors said was flawed.
Two other former presidential candidates -- Andrey Sannikau and Mikalay Statkevich -- remain in custody. Nyaklyaeu was released from jail in January and Ales Mikhalevich in February. All have been charged with organizing mass disturbances.
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