Other activists on trial in Salihorsk received fines of up to 150,000 Belarusian rubles ($70).
The activists were protesting the trial of a fellow Youth Front member, 16-year-old Ivan Shyla.
Shyla was in court on September 4 on charges of acting on behalf of an unregistered organization. As a minor, he faced a maximum sentence of six months in prison, but was released with a warning.
More than 50 opposition and human-rights campaigners attended the start of Shyla's trial, including representatives of the Belarusian Helsinki Committee, the Vyasna (Spring) human-rights group, and the United Civic Party.