Imprisoned Russian opposition leader Alexey Navalny has been sentenced to 19 more years in jail on extremism charges which he has dismissed as an attempt to silence him.

The prosecution had demanded a 20-year prison sentence, and the politician himself said that he expected a lengthy, “Stalinist” term.

Friday’s verdict marked his fifth criminal conviction; the sentence is the longest of the three he has been handed. Navalny appeared before the judge wearing his prison uniform, smiling and speaking with another defendant.

He is already serving a nine-year sentence for fraud and contempt of court in a penal colony east of Moscow. In 2021, he was also sentenced to 2.5 years in prison for a parole violation. The latest trial against Navalny has been taking place behind closed doors in the colony where he is imprisoned.

Marie Struthers, Amnesty International’s Director for Eastern Europe and Central Asia, decried the sentence as “a sinister act of political vengeance that not only targets Navalny personally but serves as a warning to state critics across the country”.

She added that the outcome of “today’s sham trial” is the latest example of the “systematic oppression of Russian civil society that has intensified since Russia’s invasion of Ukraine last year”.

The court at the IK-6 penal colony in Melekhovo, about 235km (145 miles) east of Moscow, was trying him on six separate criminal charges, including inciting and financing extremist activity and creating an extremist organisation.

In a video feed from Friday’s court hearing, Navalny could be seen wearing a black prison uniform and standing with his arms folded as he listened to the verdicts. German Foreign Minister Annalena Baerbock condemned the ruling.

“Russia’s arbitrary justice system imprisoning Alexei Navalny for another 19 years is pure injustice,” she wrote on social media. “Putin fears nothing more than standing up against war and corruption and for democracy — even from a prison cell. He will not silence critical voices with this.”

The 47-year-old is President Vladimir Putin’s fiercest foe and has exposed official corruption and organised large anti-Kremlin protests. Navalny was arrested in January 2021 upon returning to Moscow after recuperating in Germany from nerve agent poisoning that he blamed on the Kremlin.

The latest charges related to the activities of Navalny’s anti-corruption foundation and statements by his top associates. His allies said the charges retroactively criminalise all the foundation’s activities since its creation in 2011. Navalny has rejected all the charges against him as politically motivated and has accused the Kremlin of seeking to keep him behind bars for life.

In his closing statements last month, Navalny condemned Russia’s war in Ukraine. “[Russia is] floundering in a pool of either mud or blood, with broken bones, with a poor and robbed population, and around it lie tens of thousands of people killed in the most stupid and senseless war of the 21st century,” he said.