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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 — less than a month before an election that will give Putin another six years in power — brought renewed criticism and outrage from world leaders toward the Russian president who has suppressed opposition at home.

After initially allowing people to lay flowers at monuments to victims of Soviet-era repressions in several Russian cities, police sealed off some of the areas and started making arrests.

More than 100 people were detained in eight cities, including Moscow, St. Petersburg, Murmansk in the Arctic Circle, Krasnodar and Rostov-on-Don in the south of Russia, according to the OVD-Info monitoring group. Shouts of “Shame!” were heard as Moscow police rounded up more than a dozen people — including one with a sign reading “Killer” — near a memorial to political prisoners, the group said.

But there was no indication Navalny’s death would spark large protests, with the opposition fractured and now without its “guiding star,” as an associate put it.

Russia’s Federal Penitentiary Service reported Navalny felt sick after a walk Friday and lost consciousness at the penal colony in the town of Kharp, in the Yamalo-Nenets region about 1,900 kilometers (1,200 miles) northeast of Moscow. An ambulance arrived, but he couldn’t be revived; the cause of death is “being established,” it said.

Navalny had been jailed since January 2021, when he returned to Moscow to face certain arrest after recuperating in Germany from nerve agent poisoning he blamed on the Kremlin. He was later convicted three times, saying each case was politically motivated.

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

Hours after his death was reported, Navalny’s wife, Yulia Navalnaya, made a dramatic appearance at a security conference in Germany where many leaders had gathered.

She said she had considered canceling, “but then I thought what Alexei would do in my place. And I’m sure he would be here,” adding that she was unsure if she could believe the news from official Russian sources.

“But if this is true, I want Putin and everyone around Putin, Putin’s friends, his government to know that they will bear responsibility for what they did to our country, to my family and to my husband. And this day will come very soon,” Navalnaya said.

Praise for Navalny’s bravery poured in from Western leaders and others opposing Putin. Navalny’s health has deteriorated recently and the cause of death may never be known, but many of them said they held Russian authorities ultimately responsible — particularly after the deaths of many Kremlin foes.

U.S. President Joe Biden said Washington doesn’t know exactly what happened, “but there is no doubt that the death of Navalny was a consequence of something Putin and his thugs did.”

Navalny “could have lived safely in exile” but returned home despite knowing he could be imprisoned or killed “because he believed so deeply in his country, in Russia.”

In Germany, Chancellor Olaf Scholz said Navalny “has probably now paid for this courage with his life.”

Standing beside Scholz, Ukrainian President Volodymyr Zelenskyy — whose country is fending off Russia’s invasion — said: “Putin doesn’t care who dies in order for him to hold onto his position.”

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

Russia’s main state TV channel interrupted its newscast to announce the death, while other broadcasters carried only terse reports.

The Russian SOTA social media channel shared video of Navalny — reportedly in a prison courtroom on Thursday — laughing and joking with the judge via video link on one of several hearings about conditions in jail.

Navalny was moved in December from a central Russia penal colony to the “special regime” facility — the maximum security level. His allies decried the transfer to the remote Arctic colony as yet another attempt to isolate and silence Navalny.

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

In Putin’s Russia, political activists often faded amid factional disputes or went into exile after imprisonment, suspected poisonings or other 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 a physical assault or imprisonment — with intense devotion and sardonic wit. When authorities put Navalny in a tiny cell because of minor infractions — allowing access to a narrow exercise yard only in the early morning — he joked: “Few things are as refreshing as a walk in Yamal at 6:30 in the morning.”

Navalny ally Lyubov Sobol told The Associated Press that Russia’s repressive climate makes any rallies over his death risky, and “people could get long prison terms for taking part in a peaceful protest.”

In the absence of a “guiding star” like Navalny, she said, “people will have an even greater fear of repressions, seeing the government’s impunity.”

A woman laying flowers for Navalny at a Moscow memorial said he was “the last beacon of hope for anything to change, and that hope died today. So the only thing I want to do now is cry, I have no more words.” She identified herself only by her first name, Elmira, for fear of repression.

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 buying a stake in oil and gas companies to become an activist shareholder and push for transparency.

His work had pocketbook appeal to Russians’ widespread sense of being cheated, carrying stronger resonance than abstract concerns about democracy 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 surprisingly demanded his release pending appeal. A higher court later gave him a suspended sentence.

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

Navalny finished second, an impressive performance against an incumbent who was backed by Putin’s political machine and was popular for improving Moscow’s infrastructure.

Navalny’s acclaim 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 utter his name, referring to him as “that person” or similar wording, in an apparent effort to diminish his importance.

In opposition circles, Navalny was often viewed as having an overly nationalist streak for supporting the rights of ethnic Russians — he backed 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 via investigations conducted by his Fund for Fighting Corruption.

Although state-controlled TV ignored Navalny, his investigations resonated with younger Russians via YouTube and posts on his website and social media accounts. The strategy helped him reach 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 criticism of the political system under Putin. He was a galvanizing figure in protests of unprecedented size against dubious national election results and the exclusion of independent candidates.

Navalny got attention using pithy phrases and a potent image. His description of Putin’s power-base United Russia as “the party of crooks and thieves” gained instant popularity.

In 2017, after an assailant threw green-hued disinfectant in his face, seriously damaging an eye, Navalny joked that people were comparing him to the superhero the Hulk.

Much worse was to come.

While in jail in 2019 for an election protest, he was hospitalized for what authorities called an allergic reaction, but some doctors said it appeared to be poisoning.

A year later, he fell 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 before being flown to Germany for treatment.

Doctors there determined he had been poisoned with a strain of Novichok – similar to the nerve agent that nearly killed former Russian spy Sergei Skripal in 2018. Navalny was in a medically induced coma for about two weeks.

The Kremlin vehemently denied it was behind the poisoning, but Navalny challenged that with an audacious move: releasing the recording of a call he said he made to an alleged member of the Federal Security Service, or FSB, who purportedly carried out the poisoning and then tried to cover it up. The FSB called the recording a fake.

Russian authorities then announced that while 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.

Navalny and his wife nevertheless flew to Moscow on Jan. 17, 2021. On arrival, he told waiting journalists he was pleased to be back, walked to passport control and into custody.

Last month, he explained why he returned, saying: “I don’t want to give up either my country or my beliefs.”

Just over two weeks after his return, he was tried, convicted and sentenced to 2½ years in prison. That sparked massive protests reaching to Russia’s farthest corners and saw police detain over 10,000 people.

As part of a massive opposition crackdown 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 it in social media posts from prison and during his court hearings.

Less than a month after the war began, he received another nine-year term for embezzlement and contempt of court in a case he said was fabricated. Last August, he was convicted of extremism and sentenced to 19 years in prison.

When a film called “Navalny” about his story won an Academy Award for best documentary in 2023, his wife told the ceremony: “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, he is survived by a son and a daughter.