Russia has imposed a 25-year-sentence on a prominent political activist, underlining intensifying intolerance for criticism of the war in Ukraine and other dissenting opinions.

Under president Vladimir Putin, Russia for years has been cracking down on those who challenge the Kremlin. On Monday, Vladimir Kara-Murza was convicted of treason by a Moscow court and sentenced to 25 years in prison.

Kara-Murza, 41, is a historian, journalist and opposition politician who holds Russian and British passports and studied in the UK at Cambridge University. He was a close associate of Boris Nemtsov, a leading opposition figure who was assassinated near the Kremlin in 2015, and continued to speak out against Putin despite the mounting risks.

Twice, in 2015 and 2017, Kara-Murza suddenly fell ill in what he said were poisonings by the Russian security services, on both occasions falling into a coma before eventually recovering. Russian authorities denied involvement in the incidents. Kara-Murza’s lawyers say that as a result, he suffers from a serious nerve disorder called polyneuropathy.

Kara-Murza was one of a small number of opposition politicians who remained active in Russia after it invaded Ukraine in February 2022 and publicly condemned the war in defiance of new censorship laws.

Kara-Murza was arrested in April 2022, hours after CNN broadcast an interview in which he said Russia was being run by a “regime of murderers”.

He was declared a “foreign agent” and accused of spreading false information about the actions of the Russian military in Ukraine in connection with a speech he had given the previous month in the Arizona house of representatives, where he said Putin was “dropping cluster bombs on residential areas, mothers’ homes, hospitals, and schools”.

In July, he was additionally charged over his involvement with two foreign-based opposition forums that are labelled by the Russian state as “undesirable”. Finally, on October 6, he was charged with treason over public speeches he had made in Lisbon, Helsinki and Washington.

Prosecutors requested a 25-year jail term.

Kara-Murza said he had done nothing wrong and compared the proceedings to Josef Stalin’s show trials of the 1930s.

In a statement at the end of his trial, Kara-Murza said he was jailed for “many years of struggle against Putin’s dictatorship”, his criticism of the war in Ukraine and his long efforts to champion Western sanctions against Russian officials involved in human rights abuses.

“I know that the day will come when the darkness engulfing our country will dissipate,” the 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.”

Kara-Murza reacted calmly as the judge read the verdict and sentence. His lawyer, Maria Eismont, later quoted him as telling her: “My self-esteem has risen: I realised that I have done everything right. Twenty-five years is the highest appraisal that I could get for doing what I did and what I believed in, as a citizen, a patriot and a politician.”

“A quarter of a century is an ‘A+’ for your courage, consistency and honesty in your years-long work. I am infinitely proud of you, my love, and I’m always by your side,” Kara-Murza’s wife, Evgenia, who lives in the US with their children, tweeted after the verdict.

Human rights organisations and Western governments denounced the verdict and demanded his release. Amnesty International declared the 41-year-old to be a prisoner of conscience as it denounced “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.”

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

The sweeping campaign of repression is unprecedented since the Soviet era, effectively criminalising 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, who drew an anti-war sketch at school, 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.

With reporting from Reuters and AP.