Gay rights groups condemned the law.
They claimed the law violated rights to freedom of expression and prohibition of discrimination under the European Convention of Human Rights.
However, she doubted that Russian Federation would change the law, as she said that Russia's Constitutional Court could throw out European rulings. The applicants were awarded some 50,000 euros ($55,000) in total.More news: Escaped Georgia convicts who allegedly killed two guards captured in Tennessee
The Kremlin said it plans to appeal the ruling. At the same time, Peskov stressed that the law in question was aimed at banning the promotion of homosexuality precisely among underage children.
The activists were initially fined by Russian courts for appearing outside of public places, holdings signs reading "Homosexuality is normal". But their complaints - right up to the Constitutional Court - were unsuccessful.
"Above all, by adopting such laws the court found that the authorities had reinforced stigma and prejudice and encouraged homophobia, which was incompatible with the values - of equality, pluralism and tolerance - of a democratic society", the court document said.
The court ordered Russian Federation to pay the activists a combined total of €50,000 in damages.More news: Justices take on fight over partisan electoral maps
One of the claimants, Alexeyev, described the ruling as "an enormous court victory for LGBT people in Russian Federation".
Whilst homosexuality was no longer criminalised from 1993, the state still rife with anti-gay prejudice.
Gay rights activist Nikolai Alexeyev is detained and escorted away during a rally outside the Moscow mayor's office in 2013.
But President Vladimir Putin in December 2015 signed a law that allows Russia's Constitutional Court to decide whether or not to implement rulings by global courts.More news: A look at what is ahead now that Brexit talks have started
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