A prominent Russian gay rights activist was convicted on Friday of spreading "gay propaganda" among minors in the first such ruling in Russia's modern history.

Nikolai Alexeyev told the Associated Press that a city court in St. Petersburg fined him 5,000 rubles ($170) for breaching the law, which was controversially introduced by lawmakers in Russia's second-largest city in February. He pledged to appeal the decision.

Gay rights activists say the legislation could be used to ban public demonstrations.

Alexeyev was briefly detained last month after he picketed the city hall in St. Petersburg with a poster which said that "homosexuality is not a perversion."

He said the judge has not presented the grounds for her decision, and that they will only be available next week. Calls to the court went unanswered shortly after the ruling.

Homosexuality was decriminalized in Russia in 1993, but anti-gay sentiment remains strong. Alexeyev and other activists have petitioned Moscow authorities for permission to stage a gay pride parade, but have been denied. Moscow's longtime mayor Yuri Luzhkov described gay parades as "satanic"; his successor Sergei Sobyanin has said he disapproves of gay gatherings because they could offend the religious beliefs of many Russians.

Alexeyev said he would go to Russia's Constitutional Court and then to the European Court of Human Rights in Strasbourg if a higher court in St. Petersburg upholds Friday's ruling.

In an apparent effort to catch up with the initiative in St. Petersburg, the Russian parliament in late March submitted a bill calling for fines of up to 500,000 rubles ($17,000) for the same offense.

With that bill in place, authorities would not even need to explain their decision to ban any gay gatherings, critics said.

Yuri Gavrikov, head of St. Petersburg's LGBT group Equality, described the ruling as "absurd".

"There were no children where Alexeyev held his picket," he said, adding that the wording of the law that was used in the activist's case was vague.