Andżelika Borys was detained by police in Grodno, close to the Polish border, on March 23.
She was later sentenced to 15 days of confinement for organizing an "illegal mass event" and then charged as part of a criminal investigation into "inciting hatred," according to a report.
The Association of Poles in Belarus which Borys heads is not recognized by the Belarusian authorities but is nevertheless active, according to the PAP news agency.
Andrzej Poczobut, a senior member of the association, was detained in Grodno on March 25.
In March, Poland's Prime Minister Mateusz Morawiecki urged the Belarusian authorities not to "harass" Poles in that country and to stop treating them as "hostages."
The European Commission in March called on the Belarusian authorities to free and drop all charges against several leaders of the Polish ethnic minority in that country.
The EU executive said the detained members of the Association of Poles in Belarus, including Andżelika Borys and Andrzej Poczobut, should be released immediately and unconditionally.
Last month, OSCE Secretary-General Helga Schmid told Poland's president that the body will use all available diplomatic means to help Poles in Belarus.