Lyudmila Alexeyeva, a Soviet-era dissident who became a symbol of resistance in modern-day Russia as a leading rights activist, died after a long illness at the age of 91 in Moscow on Saturday.
In an extraordinary career emblematic of the country's turbulent history, she tirelessly defended human rights in the USSR from the 1950s, and continued to do so in strongman leader Vladimir Putin's Russia.
"This is a huge loss for the entire human rights movement in Russia," Mikhail Fedotov, head of the Kremlin Human Rights Council, said in a statement.
Alexeyeva died at about 1630 GMT Saturday in a Moscow hospital, he added.
"It was not the first time that she was in this hospital, doctors had already revived her several times in the most difficult of situations. But there are situations in which doctors can do nothing," he added.
"She had been struggling with illness recently, but her mind was always stronger than her body and far stronger than any disease."
Kremlin spokesman Dmitry Peskov said Putin, of whom Alexeyeva was a strong critic, had sent a message of condolences to her family.
The president "greatly appreciates Lyudmila Alexeyeva's contribution to the development of civil society in Russia and had great respect for her point of view on several issues concerning the life of the country," Russian news agencies quoted Peskov as saying.
Alexeyeva had been the leader of the Moscow Helsinki Group, one of Russia's oldest human rights organisations which she helped found in 1976.
The group lamented the loss of a "legendary, wise and humane person who remained a defender of human rights until the last moments of her life".
Russia's rights ombudswoman Tatiana Moskalkova also mourned Alexeyeva.
"For those who have appreciated democracy in the past, for those who appreciate it now, and those who will appreciate it in the future, Lyudmila Mikhaylovna has always been and will always be a symbol of honesty and the uncompromising struggle for human rights," Moskalkova told the Interfax news agency.