An 18-year-old man has become the first person punished under a new Russian law which bans adults from encouraging minors to take part in illegal protests.
Ivan Luzin was first charged after a protest in Kaliningrad last month.
Luzin told local rights group OVD-Info he had acted alone and that two 16-year-old girls with him had only been there to take photos.
He has been fined 20,000 rubles (£235; $311) and says he will appeal.
Luzin - a supporter of opposition leader Alexei Navalny - had reportedly been holding a sign in support of two jailed activists allegedly tortured by police.
"Initially, I planned to go on a single picket," he told Radio Svoboda last month..
"When the girls decided to come too, I realized that it was impossible to hold a picket. We decided to just take a picture. Everything took no more than ten minutes."
The three were later held and questioned by police. Luzin spent 24 hours in custody, while the two girls were allowed to go home.
Under the law, passed in December, offenders face fines, community service orders, or up to 15 days' prison time if found guilty.
It was proposed amid concern by the Kremlin about protests by young people, including supporters of Mr Navalny.
Russia's constitution protects the right to public protest, but a law passed in 2014 gives authorities the power to jail and fine protestors for "unauthorised" demonstrations.
In September last year, more than 800 people were detained by police after protests across the country against plans to increase retirement age.