The activists were demonstrating outside a courthouse in the town of Baranovichi to protest against the prosecution of a fellow activist, who is on trial for belonging to an unregistered group, Youth Front.
Authorities also briefly held two journalists, including a Reuters photographer, who were covering the event.
Several members of the Youth Front opposition group have been arrested and fined or sentenced to jail time in recent weeks for belonging to an unregistered group. Youth Front organizers have made repeated attempts to register the group with the Justice Ministry, but without success.
Western governments accuse the regime of Belarusian President Alyaksandr Lukashenka of harassing political opponents, muzzling the media, and rigging elections.