Jailed Russian opposition figure and outspoken Kremlin critic Alexey Navalny has died at the age of 47, the Russian prison service said.

Navalny "felt unwell after a walk" and "almost immediately" lost consciousness, the prison service said Friday. It said it was investigating his "sudden death."

Navalny had long been a thorn in the side of President Vladimir Putin, exposing corruption in high places, campaigning against the ruling United Russia party, and orchestrating some of the biggest anti-government protests seen in recent years.

He returned to Russia in 2021 from Germany, where he had been treated after being poisoned with Novichok, a Soviet-era nerve agent. Upon his return, he was swiftly arrested on charges he dismissed as politically motivated.

Navalny speaks in his office in Moscow in 2009. He used his blog and social media to expose alleged corruption in the Kremlin and Russian business.
Navalny speaks in his office in Moscow in 2009. He used his blog and social media to expose alleged corruption in the Kremlin and Russian business.
Oxana Onipko/AFP via Getty Images
Navalny speaks during a rally against parliamentary elections in Moscow in 2011. That year, he founded the Anti-Corruption Foundation to investigate corruption in the Russian government.
Navalny speaks during a rally against parliamentary elections in Moscow in 2011. That year, he founded the Anti-Corruption Foundation to investigate corruption in the Russian government.
Kirill Kudryavtsev/AFP via Getty Images
Navalny listens to opposition leader Garry Kasparov as a committee meets in January 2012 to discuss a new protest in Moscow.
Navalny listens to opposition leader Garry Kasparov as a committee meets in January 2012 to discuss a new protest in Moscow.
Anton Golubev/Reuters
Navalny and other members of Russia's opposition march in St. Petersburg, Russia, in February 2012. They were in Vladimir Putin's native city, demonstrating against his likely return to the Kremlin.
Navalny and other members of Russia's opposition march in St. Petersburg, Russia, in February 2012. They were in Vladimir Putin's native city, demonstrating against his likely return to the Kremlin.
Olga Maltseva/AFP via Getty Images
Police officers detain Navalny during a protest in Moscow in March 2012. This was after Putin had been elected president again.
Police officers detain Navalny during a protest in Moscow in March 2012. This was after Putin had been elected president again.
Alexander Nemenov/AFP via Getty Images
Navalny greets supporters from inside a car after he was released from a police station in Moscow in May 2012. He received a 15-day sentence for disobeying police during a rally.
Navalny greets supporters from inside a car after he was released from a police station in Moscow in May 2012. He received a 15-day sentence for disobeying police during a rally.
Denis Sinyakov/Reuters
Navalny lifts his son, Zakhar, while meeting with supporters in Moscow in May 2012.
Navalny lifts his son, Zakhar, while meeting with supporters in Moscow in May 2012.
Sergei Karpukhin/Reuters
A journalist films Navalny's apartment in Moscow after it was searched by police in June 2012.
A journalist films Navalny's apartment in Moscow after it was searched by police in June 2012.
Mikhail Voskresensky/Reuters
Navalny embraces his wife, Yulia, as he is released in a courtroom in Kirov, Russia, in July 2013. A court in Kirov had found Navalny and entrepreneur Petr Ofitserov guilty of embezzlement and sentenced them to five and four years in prison, respectively. They were released pending an appeal.
Navalny embraces his wife, Yulia, as he is released in a courtroom in Kirov, Russia, in July 2013. A court in Kirov had found Navalny and entrepreneur Petr Ofitserov guilty of embezzlement and sentenced them to five and four years in prison, respectively. They were released pending an appeal.
Evgeny Feldman/AP
Navalny and Ofitserov, center left, address supporters at a railway station in Moscow in July 2013. Navalny said he would push ahead with his bid to become mayor of Moscow. He finished second in the race.
Navalny and Ofitserov, center left, address supporters at a railway station in Moscow in July 2013. Navalny said he would push ahead with his bid to become mayor of Moscow. He finished second in the race.
Vasily Maximov/AFP via Getty Images
Navalny speaks to his brother and co-defendant, Oleg, as they attend the verdict announcement of their fraud trial at a court in Moscow in 2014. The Navalnys were accused of defrauding the Russian subsidiary of the French cosmetics company Yves Rocher. Alexey received a suspended sentence of three and a half years. Oleg was sentenced to three and a half years in prison and handcuffed in the courtroom. Alexey denounced it as political pressure and said he believed the conviction was politically motivated.
Navalny speaks to his brother and co-defendant, Oleg, as they attend the verdict announcement of their fraud trial at a court in Moscow in 2014. The Navalnys were accused of defrauding the Russian subsidiary of the French cosmetics company Yves Rocher. Alexey received a suspended sentence of three and a half years. Oleg was sentenced to three and a half years in prison and handcuffed in the courtroom. Alexey denounced it as political pressure and said he believed the conviction was politically motivated.
Dmitry Serebryakov/AFP via Getty Images
Navalny cut off this electronic bracelet during his house arrest in Moscow in January 2015. He was protesting what he called his illegal detention.
Navalny cut off this electronic bracelet during his house arrest in Moscow in January 2015. He was protesting what he called his illegal detention.
Aleksei Navalny Blog/Anadolu Agency/Getty Images
Navalny takes a selfie with the media during an appeal hearing in Moscow in May 2015.
Navalny takes a selfie with the media during an appeal hearing in Moscow in May 2015.
Alexander Nemenov/AFP via Getty Images
Police detain Navalny during an opposition rally in Moscow in March 2017.
Police detain Navalny during an opposition rally in Moscow in March 2017.
Evgeny Feldman for Alexey Navalny's campaign/Anadolu Agency/Getty Images
Navalny gestures during an appeal hearing at a court in Moscow in March 2017. He was sentenced to 15 days behind bars for resisting police during the opposition rally.
Navalny gestures during an appeal hearing at a court in Moscow in March 2017. He was sentenced to 15 days behind bars for resisting police during the opposition rally.
Kirill Kudryavtsev/AFP via Getty Images
Navalny poses for a photo after unknown attackers splashed him in the face with an antiseptic green dye in April 2017. The attack caused vision damage in one eye.
Navalny poses for a photo after unknown attackers splashed him in the face with an antiseptic green dye in April 2017. The attack caused vision damage in one eye.
Evgeny Feldman/Navalny Campaign via AP
Navalny leaves a police station in Moscow in September 2017. He was detained earlier in the day ahead of a rally, and he was to face a court hearing in Moscow over repeatedly violating a law on organizing public meetings.
Navalny leaves a police station in Moscow in September 2017. He was detained earlier in the day ahead of a rally, and he was to face a court hearing in Moscow over repeatedly violating a law on organizing public meetings.
Maxim Zmeyev/AFP via Getty Images
From an office in Moscow, Navalny speaks during an interview with the news organization Agence France-Presse in January 2018. A Moscow court had ordered the closure of his Anti-Corruption Foundation. The foundation's lawyers were appealing.
From an office in Moscow, Navalny speaks during an interview with the news organization Agence France-Presse in January 2018. A Moscow court had ordered the closure of his Anti-Corruption Foundation. The foundation's lawyers were appealing.
Mladen Antonov/AFP via Getty Images
Navalny attends a rally in Moscow in January 2018.
Navalny attends a rally in Moscow in January 2018.
Evgeny Feldman/AP
Navalny monitors voting at the Anti-Corruption Foundation office in Moscow in March 2018.
Navalny monitors voting at the Anti-Corruption Foundation office in Moscow in March 2018.
Maxim Zmeyev/AFP via Getty Images
Navalny, along with other opposition supporters in Moscow, pays tribute to the victims of a shopping mall fire in Siberia in March 2018.
Navalny, along with other opposition supporters in Moscow, pays tribute to the victims of a shopping mall fire in Siberia in March 2018.
Mladen Antonov/AFP via Getty Images
Navalny attends a hearing at a court in Moscow in June 2019. Navalny was appearing in court after taking part in an unauthorized protest.
Navalny attends a hearing at a court in Moscow in June 2019. Navalny was appearing in court after taking part in an unauthorized protest.
Vasily Maximov/AFP via Getty Images
Navalny casts his vote at a polling station in Moscow in September 2019.
Navalny casts his vote at a polling station in Moscow in September 2019.
Vasily Maximov/AFP via Getty Images
An air ambulance takes off from the Russian city of Omsk in 2020, carrying Navalny to Germany after he was poisoned with the chemical nerve agent Novichok. He arrived comatose at a hospital in Berlin but would recover. <a href="index.php?page=&url=https%3A%2F%2Fwww.cnn.com%2F2020%2F12%2F14%2Feurope%2Frussia-navalny-agents-bellingcat-ward%2Findex.html" target="_blank">A joint investigation by CNN and the group Bellingcat</a> implicated the Russian Security Service (FSB) in Navalny's poisoning. The investigation found that the FSB toxins team of about six to 10 agents trailed Navalny for more than three years. Russia denied involvement.
An air ambulance takes off from the Russian city of Omsk in 2020, carrying Navalny to Germany after he was poisoned with the chemical nerve agent Novichok. He arrived comatose at a hospital in Berlin but would recover. A joint investigation by CNN and the group Bellingcat implicated the Russian Security Service (FSB) in Navalny's poisoning. The investigation found that the FSB toxins team of about six to 10 agents trailed Navalny for more than three years. Russia denied involvement.
Dimitar Dilkoff/AFP via Getty Images
Navalny is surrounded by his wife, Yulia, and two children, Zakhar and Daria, while on a hospital bed in Berlin in September 2020. On social media, he said he was breathing on his own without medical support.
Navalny is surrounded by his wife, Yulia, and two children, Zakhar and Daria, while on a hospital bed in Berlin in September 2020. On social media, he said he was breathing on his own without medical support.
Alexey Navalny/Instagram/Anadolu Agency via Getty Images
Navalny takes his seat on a Moscow-bound plane before taking off from Berlin in January 2021. Navalny was detained by police moments after landing in Russia.
Navalny takes his seat on a Moscow-bound plane before taking off from Berlin in January 2021. Navalny was detained by police moments after landing in Russia.
Kirill Kudryavtsev/AFP via Getty Images
Navalny appears on a TV screen during a court hearing of his appeal in Moscow in January 2021. A Moscow court sentenced Navalny to prison for more than two and a half years for violating probation terms while he was in Germany.
Navalny appears on a TV screen during a court hearing of his appeal in Moscow in January 2021. A Moscow court sentenced Navalny to prison for more than two and a half years for violating probation terms while he was in Germany.
Alexander Zemlianichenko/AP
Navalny appears in court for a hearing in Moscow in February 2021.
Navalny appears in court for a hearing in Moscow in February 2021.
Moscow Court Press Service/Anadolu Agency via Getty Images
In this image, taken from a TV screen during a live broadcast of a court hearing, Navalny speaks with his wife, Yulia, on the first day of his new trial in Pokrov, Russia, in February 2022. Navalny was sentenced to nine years in a maximum-security jail, according to the Russian state-owned news agency Tass, after being convicted on fraud charges over allegations that he stole from his Anti-Corruption Foundation.
In this image, taken from a TV screen during a live broadcast of a court hearing, Navalny speaks with his wife, Yulia, on the first day of his new trial in Pokrov, Russia, in February 2022. Navalny was sentenced to nine years in a maximum-security jail, according to the Russian state-owned news agency Tass, after being convicted on fraud charges over allegations that he stole from his Anti-Corruption Foundation.
Alexander Nemenov/AFP via Getty Images
Navalny appears via video during his appeal hearing in May 2022.
Navalny appears via video during his appeal hearing in May 2022.
Alexander Nemenov/AFP via Getty Images
Navalny appears on video from the Arctic penal colony where he was serving prison time in January 2024. He died in February. Navalny "felt unwell after a walk" and "almost immediately" lost consciousness, the Russian prison service said Friday. It said it was investigating his "sudden death."
Navalny appears on video from the Arctic penal colony where he was serving prison time in January 2024. He died in February. Navalny "felt unwell after a walk" and "almost immediately" lost consciousness, the Russian prison service said Friday. It said it was investigating his "sudden death."
Alexander Zemlianichenko/AP