Lyudmila Alexeyeva, the renowned Russian human rights activist and Kremlin critic, has died in a Moscow hospital at the age of 91 after a serious illness.

The Presidential Council for Civil Society and Human Rights confirmed her death in a statement late Saturday.

"This is a terrible loss for the entire Russian human rights movement," council chairman Mikhail Fedotov said.

Alexeyeva was the country's best known rights activist and had been working to secure the release of imprisoned political activist Lev Ponomaryov until shortly before her death.

"She remained a human rights activist till the end," Fedotov said.

She joined the rights movement in 1966, according to the Russian news agency Interfax, when she began campaigning for the release of two dissident writers who had been jailed after publishing their work abroad to bypass Soviet censors.

Ten years later, together with the Nobel Peace Prize laureate Andrei Sakharov, she founded the Moscow Helsinki Group, which was committed to defending human rights. In 1977 Soviet authorities forced her into exile.

She settled in the United States but returned to Russia in 1993 after the dissolution of the Soviet Union.

Her work was recognized with numerous awards throughout her career, including France's highest order of merit, the Legion of Honour, in 2004.

President Vladimir Putin, who had visited her in her Moscow flat on her 90th birthday, paid tribute to her on Saturday, according to his spokesman.

"The president appreciates Lyudmila Alexeyeva's contribution to the development of civil society in Russia," Dmitry Peskov told Interfax news agency on Saturday night.

The president highly respected her positions on many issues, he said. Putin had also sent a personal letter of condolence to her family.

The Interior Ministry said Alexeyeva would be remembered as an example of charity, wisdom and boundless dedication.

Thorbjorn Jagland, secretary general of the Council of Europe, called her the "world matriarch of human rights defenders" and said she would be "missed sorely by all of us."