A Russian court convicted a top opposition activist of treason on Monday for publicly denouncing Moscow’s war in Ukraine and sentenced him to 25 years in prison. It was the latest move in the Kremlin’s relentless crackdown on anyone who dares to criticize the invasion.

Vladimir Kara-Murza, Jr., an activist and journalist who twice survived poisonings he blamed on the Kremlin, has rejected the charges against him as punishment for standing up to Russian President Vladimir Putin and likened the proceedings to the show trials during the rule of Soviet dictator Josef Stalin.

Human rights organizations and Western governments denounced the verdict and demanded his release. Amnesty International declared the 41-year-old a prisoner of conscience.

The charges against Kara-Murza, who has been behind bars since his arrest a year ago, stem from a March 2022 speech to the Arizona House of Representatives in which he denounced Russia’s invasion of Ukraine and other speeches abroad.

Days after that invasion, the country adopted a law criminalizing spreading “false information” about its military. Authorities have used the law to stifle criticism of what the Kremlin calls “a special military operation.”

The sweeping campaign of repression is unprecedented since the Soviet era, effectively criminalizing independent reporting on the conflict and any public criticism of the war.

Last month, a Russian court convicted a father over social media posts critical of the war and sentenced him to two years in prison. His 13-year-old daughter was sent to an orphanage. Days later, Russia’s security service arrested Evan Gershkovich, an American reporter for The Wall Street Journal, on espionage charges.

In a statement at the end of his trial, Kara-Murza said that he was jailed for “many years of struggle against Putin’s dictatorship.”

“I know that the day will come when the darkness engulfing our country will dissipate,” Kara-Murza, a father of three, told the court in remarks that were posted on his Twitter account. “This day will come as inevitably as spring comes to replace even the frostiest winter.”

Survived Poisonings in Past

Kara-Murza was an associate of Russian opposition leader and fierce Putin critic Boris Nemtsov, who was assassinated near the Kremlin in 2015. Kara-Murza survived poisonings in 2015 and 2017 that he blamed on the Kremlin. Russian officials have denied responsibility.

Another prominent opposition figure, Ilya Yashin, was sentenced to 8½ years in prison late last year on charges of discrediting the military.

Amnesty International denounced Kara-Murza’s sentence as “yet another chilling example of the systematic repression of civil society, which has broadened and accelerated under the Kremlin since Russia’s invasion of Ukraine last year.”

“The so-called ‘crimes’ Vladimir Kara-Murza was tried for — speaking out against the invasion and advocacy on behalf of victims of human rights violations — are in fact acts of outstanding bravery,” Amnesty’s Russia Director, Natalia Zviagina, said in a statement. “This verdict wrongly conflates human rights activism with ‘high treason’ and is reminiscent of Stalin-era repression.”

The group declared Kara-Murza a prisoner of conscience, convicted solely for his political beliefs, and demanded his immediate and unconditional release.

British Foreign Secretary James Cleverly also condemned the sentencing of Kara-Murza, who also has British citizenship.

“Vladimir Kara-Murza bravely denounced Russia’s invasion of Ukraine for what it was — a blatant violation of international law and the U.N. Charter,” Cleverly said in a statement.

The Foreign Office said it summoned Russian Ambassador Andrey Kelin over the conviction. The British government previously sanctioned the judge who presided over the trial for human rights violations in another case and said it would consider taking further action to hold people accountable for Kara-Murza’s case.

The United Nations’ Human Rights Chief Volker Türk criticized the sentence as “another blow to the rule of law and civic space in the Russian Federation,” and Germany’s Foreign Ministry called for his release.

Kremlin spokesman Dmitry Peskov refused to comment on the sentence.

Kara-Murza’s health has deteriorated in custody, leading to the development of polyneuropathy — disease of or damage to nerves — in both his feet, according to his lawyers.