Belarusian lawyer Maxim Znak was taken from his office by masked people in plain clothes on Wednesday, the opposition Coordination Council said.
Znak is a member of the council board and a prominent critic of President Alexander Lukashenko.
The Belarusian police initially said they were not aware of the alleged detention, but media reports also indicate that Znak's office was being searched.
Znak was a lawyer for Viktor Barbariko, who intended to stand against Lukashenko in the August elections before he was imprisoned in June.
Maria Kolesnikova, another opposition figure, was allegedly snatched from the street in Minsk on Monday and bundled into a van before reappearing at the Ukrainian border on Tuesday.
The council said they could not reach Znak on the phone, but that he texted the word "masks" to colleagues.
Znak was one of the few prominent opposition candidates in Belarus who has not yet been imprisoned.
Znak is a member of the Coordination Council set up by opposition candidate Sviatlana Tsikhanouskaya who ran against Lukashenko in the August 9 presidential election.
Tsikhanouskaya called on Russia to "support the Belarusian people" in a video message on Wednesday, describing Russia as leading a "propaganda campaign" against the protest movement.
Hours later, Lukashenko announced he would visit Russia next week.
Protesters have disputed the official election results that saw Lukashenko claim a sixth term in office with 80% of the vote. Hundreds of thousands of people have taken to the streets over the last month, with tens of thousands protesting against the president once again on Sunday.
Authorities arrested more than 600 people over the weekend. Rights groups have widely reported hundreds of counts of torture. Police said they detained a further 121 people at protests on Tuesday.