The activist, Tadeusz Gawin, denies the allegation.
The former head of the Union of Poles was originally sentenced to jail time for organizing what the Belarus government called an unsanctioned rally.
The rally was a meeting between Polish activists and Donald Tusk, the deputy speaker of the Polish parliament.
Gawin, a leader of Belarus' ethnic Polish minority, organized the meeting.
Poland has criticized Belarus for allegedly mistreating ethnic Poles.
Belarus President Alexander Lukashenko denies the claims and has himself accused Poland for rallying ethnic Poles against him.