Aleksei A. Navalny, an anticorruption activist who for more than a decade led the political opposition in President Vladimir V. Putin’s Russia, died Friday in a prison inside the Arctic Circle, according to Russian authorities.

His death was announced by Russia’s Federal Penitentiary Service, which said that Mr. Navalny, 47, lost consciousness on Friday after taking a walk in the prison where he was moved late last year. He was last seen on Thursday, when he had appeared in a court hearing via video link, smiling behind the bars of a cell and making jokes.

Leonid Volkov, Navalny’s longtime chief of staff, said he was not yet ready to accept the news that Mr. Navalny was dead. “We have no reason to believe state propaganda,” Volkov wrote on the social platform X. “If this is true, then it’s not ‘Navalny died,’ but ‘Putin killed Navalny,’ and only that. But I don’t trust them one penny.”

Mr. Navalny had been serving multiple sentences that would most likely have kept him in prison until at least 2031 on charges that his supporters say were largely fabricated in an effort to muzzle him. Despite increasingly harsh conditions, including repeated stints in solitary confinement, he maintained a presence on social media, while members of his team continued to publish investigations into Russia’s corrupt elite from exile.

Mr. Navalny was given a three-and-a-half-year prison sentence in February 2021 after returning to Russia from Germany, where he had been recovering from being poisoned with a nerve agent the previous August. In March 2022, he received a nine-year sentence for embezzlement and fraud in a trial that international observers denounced as “politically motivated” and a “sham.” And in August 2023, he was sentenced to 19 years in prison for “extremism.”

Mr. Navalny had effectively returned from the dead after his 2020 poisoning and had conducted multiple hunger strikes to improve his treatment, with many of his supporters believing him to be all but invincible.

During his detention, Mr. Navalny was repeatedly placed in solitary confinement, and complained about severe illnesses. In December, he disappeared for three weeks during his transfer to a penal colony 40 miles north of the Arctic Circle.

Mr. Navalny was an unflinching critic of Mr. Putin, a former K.G.B. officer whom he accused of corruptly skimming the country’s oil profits to enrich his friends and entourage in the security services. Mr. Putin’s political party, he said, was a party of “swindlers and thieves,” and he accused the president of trying to turn Russia into a “feudal state.”

Mr. Navalny was known for his innovative tactics in fighting corruption and promoting democracy. Defying expectations, he cannily used street politics and social media to build a tenacious opposition movement even after much of the independent news media in Russia was squelched and other critics were driven into exile or killed in unsolved murders.

Mr. Navalny was known for his innovative tactics in fighting corruption and promoting democracy. Defying expectations, he cannily used street politics and social media to build a tenacious opposition movement even after much of the independent news media in Russia was squelched and other critics were driven into exile or killed in unsolved murders.

In the years before Russia invaded Ukraine, many of Mr. Navalny’s associates, and in some cases their relatives, were arrested or forced into exile.

Before his reported death, he was the most prominent critic of Mr. Putin still standing in Russia, at a time when the president has engineered a path to remain in power until at least 2036.

Mr. Navalny was thought to have been physically attacked at least twice before: a suspected poisoning attempt when he was in jail in 2019; and an assault in 2017 in which someone threw a green liquid in his face, nearly blinding him.

He had spoken openly of the possibility that he might be assassinated.

“I’m trying not to think about it a lot,” he said in an interview with CBS News in 2017. “If you start to think about what kind of risks I have, you cannot do anything.”

Mr. Navalny became violently ill and fell into a coma on Aug. 20, 2020, shortly after boarding a flight from Siberia, where he had met with opposition candidates for local office.

He said the poison had been planted in his underwear at his hotel sometime before he boarded the plane. The flight made an emergency landing in the Russian city of Omsk, where doctors for two days resisted his wife’s pleas that he be transferred to Germany for treatment.

Mr. Navalny was eventually evacuated to Berlin on an air ambulance flight arranged by the foundation of a movie producer based there. A little more than a week later, the German government announced that he had been poisoned with a nerve agent from the highly potent Novichok family of toxins. The evidence, German officials said, was “unequivocal.”

Russian officials had previously deployed a low-level campaign of harassment against Mr. Navalny. He was frequently arrested and jailed for short spells, usually for minor offenses related to protesting without a parade permit.

Mr. Putin barely mentioned Mr. Navalny’s name, and the state news media steadfastly ignored him throughout his decade-long anticorruption campaign. Yet Mr. Navalny, a young, scrappy politician, found a base of support in the Russian middle class, and that clearly irritated the Kremlin.

Dismissing him as an unpatriotic gadfly, the Kremlin at times seemed willing to overlook his criticisms to give Mr. Putin the veneer of running a government that tolerated dissent. The short detentions allowed the Russian authorities to keep Mr. Navalny out of sight for important events, like organized protests, while escaping criticism for harsh treatment that might make him a martyr.

Despite the attacks and the jail terms, Mr. Navalny persevered, he said, out of a desire to change the course of his country and not let down the people who worked with him. He was angry at what he called Mr. Putin’s self-dealing inner circle and the security services that protected it.

“I do this because I hate these people,” he said in an interview with The New York Times in 2011, before he rose to prominence.

The New York Times Services.