Belarusian opposition politician Maxim Znak was taken from his office by masked people wearing plain clothes, the Interfax news agency cited his allies as saying on Wednesday.
The Belarusian police said they were not aware of Znak's alleged detention, the RIA news agency reported.
On Tuesday, prominent opposition leader Maria Kolesnikova tore up her passport to avoid being forced to cross the border into Ukraine after she was snatched from the street in Minsk, two of her allies said.