The activists were Brest to attend a book reading by youth activist Pavel Sevyarynets that was organized by the regional branch of the Belarusian Popular Front.
The Youth Front (Malady Front) leader was to discuss three of his recently published books, one of which was written while he was serving a restricted-freedom sentence for organizing protests after Belarus's 2004 referendum. The contentious referendum freed up President Alyaksandr Lukashenka to run for a third presidential term.
Sevyarynets told RFE/RL's Belarusian Service that a group of plainclothes police showed up 30 minutes into the event and detained all in attendance.
He claimed that those detained were taken to a police station, where they were charged with participating in an unsanctioned meeting.
Sevyarynets said the group was detained for five to seven hours before being released. Except for the minors, those detained must appear before a court on August 22 to face those charges, he said.
Police also accused the detainees of using illegally smuggled Ukrainian alcohol at the meeting, Sevyarynets said, and told them that counterfeit money bearing Lukashenka's likeness was found at the site.
Sevyarynets said he plans to press charges of police harassment related to the incident.