Alexei Navalny, who crusaded against official corruption and staged massive anti-Kremlin protests as President Vladimir Putin's fiercest foe, died Friday in the Arctic penal colony where he was serving a 19-year sentence, Russia's prison agency said. He was 47.

The stunning news of Navalny's death — less than a month before an election that will give Putin another six years in power — brought renewed criticism and outrage directed at the Kremlin leader who has cracked down on all opposition at home.

Navalny felt unwell after a walk, according to the Federal Penitentiary Service, and lost consciousness. An ambulance arrived but paramedics failed to revive him. The service said the cause of death was "being established."

Navalny had been behind bars since January 2021, when he returned to Moscow after recuperating in Germany from nerve agent poisoning that he blamed on the Kremlin. Since then, he received three prison sentences, all of which he rejected as politically motivated.

Praise for Navalny's bravery poured in from Western leaders and others who have opposed Putin's rule. The opposition leader's health has deteriorated recently and the cause of death remains unknown, but many world leaders said they held Russian authorities ultimately responsible for his death.

"His death in a Russian prison and the fixation and fear of one man only underscores the weakness and rot at the heart of the system that Putin has built. Russia is responsible for this," U.S. Secretary of State Antony Blinken said while at a conference in Germany.

German Chancellor Olaf Scholz said Navalny "has probably now paid for this courage with his life."

Kremlin spokesman Dmitry Peskov said Putin was informed of Navalny's death. The opposition leader's spokeswoman Kira Yarmysh said on X, the platform formerly known as Twitter, that the team had no confirmation yet.

Shortly after the death was reported, the Russian SOTA social media channel shared images of the opposition politician reportedly in court yesterday. In the footage, Navalny is seen standing up and is laughing and joking with the judge via video link.

Navalny was moved in December from a prison in central Russia to a "special regime" penal colony — the highest security level of prisons in the country — above the Artic Circle.

His allies decried the transfer to a colony in the town of Kharp, in a region about 1,900 kilometers (1,200 miles) northeast of Moscow, as yet another attempt to force Navalny into silence.

Before his arrest, Navalny campaigned against official corruption, organized major anti-Kremlin protests and ran for public office.

In Putin's Russia, political opponents often faded amid factional disputes or went into exile after imprisonment, suspected poisonings or other heavy repression. But Navalny grew consistently stronger and reached the apex of the opposition through grit, bravado and an acute understanding of how social media could circumvent the Kremlin's suffocation of independent news outlets.

He faced each setback — whether it was a physical assault or imprisonment — with an intense devotion, confronting dangers with a sardonic wit. That drove him to the bold and fateful move of returning from Germany to Russia and certain arrest.

Recently, prison authorities have repeatedly put Navalny in a tiny cell to punish him for minor infractions. Last month, he said in a statement relayed on social media that he was placed in the cell after officials accused him of refusing to "introduce himself in line with protocol."

Placement in the small cell means that prisoners are only allowed to walk outside in a narrow concrete prison yard at 6:30 a.m. "Few things are as refreshing as a walk in Yamal at 6:30 in the morning," he wrote.

Navalny was born in Butyn, about 40 kilometers (25 miles) outside Moscow. He received a law degree from People's Friendship University in 1998 and did a fellowship at Yale in 2010.

He gained attention by focusing on corruption in Russia's murky mix of politicians and businesses; one of his early moves was to buy a stake in Russian oil and gas companies to become an activist shareholder and push for transparency.

By concentrating on corruption, Navalny's work had a pocketbook appeal to Russians' widespread sense of being cheated, and he carried stronger resonance than more abstract and philosophical concerns about democratic ideals and human rights.

He was convicted in 2013 of embezzlement on what he called a politically motivated prosecution and was sentenced to five years in prison, but the prosecutor's office later surprisingly demanded his release pending appeal. A higher court later gave him a suspended sentence.

The day before the sentence, Navalny had registered as a candidate for Moscow mayor. The opposition saw his release as the result of large protests in the capital of his sentence, but many observers attributed it to a desire by authorities to add a tinge of legitimacy to the mayoral election.

Navalny finished second, an impressive performance against the incumbent who had the backing of Putin's political machine and was popular for improving the capital's infrastructure and aesthetics.

Navalny's popularity increased after the leading charismatic politician, Boris Nemtsov, was shot and killed in 2015 on a bridge near the Kremlin.

Whenever Putin spoke about Navalny, he made it a point to never mention the activist by name, referring to him as "that person" or similar wording, in an apparent effort to diminish his importance.

Even in opposition circles, Navalny was often viewed as having a overly nationalist streak for supporting the rights of ethnic Russians — he supported the annexation of the Crimean Peninsula by Moscow in 2014 although most nations viewed it as illegal — but he was able to mostly override those reservations with the power of investigations conducted by his Fund for Fighting Corruption.

Although state-controlled TV channels ignored Navalny, his investigations resonated with younger Russians via YouTube videos and posts on his website and social media accounts. The strategy helped him reach into the hinterlands far from the political and cultural centers of Moscow and St. Petersburg and establish a strong network of regional offices.

His work broadened from focusing on corruption to wholescale criticism of the political system under Putin. He was a central galvanizing figure in protests of unprecedented size against dubious national election results and the exclusion of independent candidates.

Navalny understood that he could get attention with a pithy phrase and potent image. His description of Putin's power-base United Russia as "the party of crooks and thieves" gained instant popularity; a lengthy investigation into then-Prime Minister Dmitry Medvedev's lavish country getaway boiled down to the complex's well-appointed duck house. Soon, comical yellow duck toys became a popular way to mock the premier.

In 2017, after an assailant threw green-hued disinfectant in his face, seriously damaging one of his eyes, Navalny joked in a video blog that people were comparing him to the comic book character The Hulk.

Much worse was to come.

While serving a jail sentence in 2019 for involvement in an election protest, he was taken to the hospital with an illness that authorities said was an allergic reaction, but some doctors said it appeared to be poisoning.

A year later, on Aug. 20, 2020, he became severely ill on a flight to Moscow from the Siberian city of Tomsk. The plane made an emergency landing in the city of Omsk, where he spent two days in a hospital while supporters begged doctors to allow him to be taken to Germany for treatment.

Once in Germany, doctors determined he had been poisoned with a strain of Novichok – similar to the nerve agent that nearly killed former Russian spy Sergei Skripal and his daughter in England in 2018 and resulted in the death of another woman.

Navalny was in a medically induced coma for about two weeks. His first communication while recovering showed his defiant wit — an Instagram post saying that breathing on one's own is "a remarkable process that is underestimated by many. Strongly recommended."

The Kremlin vehemently rejected it was behind the poisoning, but Navalny challenged the denial with an audacious move — essentially a deadly serious prank phone call. He released the recording of a call he said he made to an alleged member of a group of officers of the Federal Security Service, or FSB, who purportedly carried out the poisoning and then tried to cover it up. The FSB dismissed the recording as fake.

Russian authorities then raised the stakes, announcing that during his time in Germany, Navalny had violated the terms of a suspended sentence in one of his convictions and that he would be arrested if he returned home.

Remaining abroad wasn't in his nature. Navalny and his wife boarded a plane for Moscow on Jan. 17, 2021. On arrival, he told waiting journalists that he was pleased to be back, walked to passport control and into custody. In just over two weeks, he was tried, convicted and sentenced to 2½ years in prison.

The events sparked massive protests that reached to Russia's farthest corners and saw more than 10,000 people detained by police.

As part of a massive crackdown against the opposition that followed, a Moscow court in 2021 outlawed Navalny's Foundation for Fighting Corruption and about 40 regional offices as extremist, a verdict that exposed members of his team to prosecution.

When Putin sent troops to invade Ukraine on Feb. 24, 2022, Navalny strongly condemned the war in social media posts from prison and during his court appearances.

Less than a month after the start of the war, he was sentenced to an additional nine-year term for embezzlement and contempt of court in a case he and his supporters rejected as fabricated. The investigators immediately launched a new probe, and in August 2023 Navalny was convicted on charges of extremism and sentenced to 19 years in prison.

After the verdict, Navalny said he understood that he was "serving a life sentence, which is measured by the length of my life or the length of life of this regime."

A documentary called "Navalny" about his story won an Academy Award for best documentary in March 2023.

Navalny's wife spoke at the award ceremony, saying: "My husband is in prison just for telling the truth. My husband is in prison just for defending democracy. Alexei, I am dreaming of the day you will be free and our country will be free."

Besides his wife, Navalny is survived by a son and a daughter.