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.

In recent years, he has been a vocal opponent of President Vladimir Putin's government.

"Putin's justice in all its glory," opposition activist Ilya Yashin said on Twitter.