Human rights activist Yelena Georgievna Bonner has died at the age of 88.
Jewish Agency chairman, and former Soviet refusenik Natan Sharansky said: "The Jewish world and the state of Israel lost a stalwart and valiant defender today.
"The international community of human rights activists, supporters of democracy, dissidents and political prisoners lost one of their most important leaders. While Andrei Sakharov was the heart of the movement to defeat the evils in the Soviet system, Yelena Bonner was the catalyst who led us to action.
"On a personal note, my family and I feel that we have lost a dear friend, one who stood by us and did not leave our side even when we stood at the gates of Hell. Her cherished memory will remain in my heart forever."
Mrs Bonner was the wife of the 1975 Nobel Peace prize-winner, nuclear physicist and human rights activist Andrei Sakharov. Credited with hastening the end of communism, her and her husband were two of the Soviet Union's most vocal human rights champions.
After criticising the Soviet invasion of Afghanistan in 1979, Mr Sakharov was sent into internal exile in Gorky – now Nizhny Novgorod. Ms Bonner, who stayed with her husband during his exile, was later arrested herself for helping him communicate with the West.
Mrs Bonner's Jewish parents were victims of Stalin's purges, her father was arrested in May 1937 and executed the following year and her mother spent almost a decade in a labour camp.
When Mrs Bonner refused to condemn them as enemies of the people, Bonner joined them in disgrace and was expelled from the Young Communist League. She was also forbidden to attend university and studied Russian literature at evening classes at the Herzen Institute in Leningrade.
However, in 1941, when Hitler invaded the Soviet Union, Bonner volunteered for the front as a nurse. Her position was short-lived as she soon suffered a serious head injury that almost resulted in her losing her sight.
She married Mr Sakharov in 1972, after meeting him two years earlier at a Soviet dissident trial.
She accepted the Nobel Peace Prize on behalf of Mr Sakharov in Oslo in 1975 when he was denied an exit visa. After his death in 1989, she continued working in Russia before moving to America in 2003.
Mrs Bonner will buried next to her husband in Moscow's Vostryakovo cemetery.