A special-task police unit broke up a meeting of the Youth Front in a private apartment on February 4.
The group's spokeswoman, Lyudmila Atakulava, told the AFP news agency that activists were meeting to "outline" tactics for the "spring political campaign." She said there were minors among those detained.
All the detainees except two -- Aleh Korban and Zmitser Khvedaruk -- were released later in the evening on February 4.
Opposition activists opposed to the authoritarian rule of President Alyaksandr Lukashenka are frequently detained in Belarus. Under the country's Criminal Code, members of unregistered groups can face up to two years in jail.
(with material from agency reports)