A Moscow court on Wednesday sentenced a 77-year-old rights activist to 25 days in jail for calling for protests against a growing crackdown on young people.
Lev Ponomaryov, one of Russia's most respected activists, told AFP that the powerful FSB security service was behind his detention.
"They are taking revenge against me because I am waging a war against the FSB," the head of the For Human Rights movement told AFP by phone as he was being driven to a detention centre.
"The country is gradually inching towards mass political repressions," he said, referring to the peak of Stalin-era purges.
A spokeswoman for Moscow's Tverskoi district court said the activist had been convicted of a repeat violation of rules around mass demonstrations.
In late October, Ponomaryov made a public call for Russians to take part in an unsanctioned rally in Moscow to protest a growing crackdown on young people including teenagers suspected of extremism.
Eighteen people were detained for taking part in the rally, near the building housing the FSB, successor to the Soviet-era KGB.
In a case denounced by rights activists, a group of youngsters has been accused of seeking to topple the authorities and creating an "extremist organisation" that was infiltrated by security services.
In a statement, Ponomaryov's movement denounced his arrest and sentencing as "absolutely illegal". It also suggested he was jailed because he was helping to prepare a new opposition rally set for mid-December.
Supporters expressed their dismay at the jailing of the elderly activist, a former physics professor who rose to prominence as a rights activist in the late 1980s and served in Russia's first post-Soviet parliament.
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