According to a statement from the Federal Penitentiary Service, Alexei Navalny, a prominent Russian opposition leader and anti-corruption activist, passed away in prison on February 16. After a short walk at the Arctic penal colony where he was serving a three-decade term on charges of extremism, he collapsed and never regained consciousness.

Russia state media initially reported that Navalny had suffered a blood clot, but the story changed when Navalny’s mother and lawyer arrived at the prison on Saturday. They were informed that he had experienced “sudden death syndrome,” which is a term for different cardiac syndromes that cause cardiac arrest, but the cause of death has not been established. Until the investigation is complete, his family and lawyer have been denied access to his body, which has caused Navalny’s supporters to accuse Russian authorities of deliberately withholding his body to conceal indications of how he died if he was murdered. Although more than 12,000 people have submitted requests to the Russian government asking for Navalny’s body to be handed over to his family, it remains unclear when and if the authorities will release his body.

As the news spread worldwide, many world leaders have blamed Navalny’s death on President Vladimir Putin. On Friday, President Joe Biden stated that he was “not surprised and outraged” while blaming Putin for Navalny’s death.

“The fact of the matter is, Putin is responsible. Whether he ordered it, he’s responsible for the circumstance. It’s a reflection of who he is. It cannot be tolerated,” Biden expressed, reaffirming his position that Putin was ultimately responsible for his death.

Meanwhile, at least 400 people were detained in over a dozen Russian cities by Saturday night while paying tribute to Navalny according to the OVD-Info, a Russian human rights organization that monitors and reports politically-motivated arrests and prosecutions in Russia. In St. Petersburg, over 200 arrests were made, including Grigory Mikhnov-Voitenko, a priest of the Apostolic Orthodox Church. He had announced plans for a memorial service for Navalny on social media, but was detained on Saturday morning according to OVD-Info. In several videos, police in riot gear were seen roughly arresting and threatening the people to force them to disperse. In a video recorded in Moscow, dozens of police officers can be seen stationed near the monument to Victims of Political Repression, which honors victims harmed by the police state of the Soviet Union.

For more than a decade, Navalny led the political opposition against President Putin’s Russia, and his vocal criticism of the Kremlin remained consistent despite the danger he was aware he was in. Navalny fought against official corruption and staged massive anti-Kremlin protests as one of Kremlin’s fiercest critics while receiving multiple prison terms, surviving the Novichok nerve agent, and being physically assaulted numerous times. The death of Alexei Navalny dealt a severe blow to Russian opposition, and he is the latest to join the long list of fallen Russian critics of Putin.