MOSCOW, August 9. /TASS/. Soviet dissident, biologist Sergey Kovalev has passed away at the age of 91, his son Ivan announced on his Facebook page on Monday.

"My father died in his sleep today, in the early morning of August 9," he wrote.

The press service of the Yabloko party (Kovalev was a member) likewise reported that he had passed away.

Kovalev was actively involved in social activism since the 1950s, he advocated human rights in the USSR. In 1975, he was sentenced to seven years behind bars on charges of anti-Soviet propaganda. After returning to Moscow in March 1990, he was elected to the Supreme Soviet of the Russian Soviet Federative Socialist Republic (RSFSR).

Kovalev was one of the authors of the 1991 Declaration of Human and Civil Rights. He was also a member of the Russian State Duma (lower house of parliament), served as Russia’s Human Rights Commissioner in 1994-1995. He also opposed the federal forces’ operation in Chechnya. Kovalev was a member of the Yabloko party from 2006 until his death.