A Russian court has rejected an appeal by jailed opposition leader Alexey Navalny against a 19-year prison sentence on extremism charges, according to Russian state media RIA, in the latest crackdown on the outspoken Kremlin critic.

Navalny was sentenced in August, after he was found guilty of creating an extremist community, financing extremist activities and numerous other crimes.

He is already serving sentences of 11-and-a-half years in a maximum security facility on fraud and other charges he denies. Navalny appeared at the hearing on Tuesday via video link from a penal colony in the Vladimir region, east of Moscow.

Supporters of Navalny claim his arrest and incarceration are a politically motivated attempt to stifle his criticism of Russian President Vladimir Putin.

Navalny’s team said the hearing was moved behind closed doors, after a letter from Russia’s Ministry of Internal Affairs described potential unspecified danger to the participants in an open trial.

“The main reason why these processes are taking place behind closed doors is the restriction of my and Kholodny’s rights,” Navalny told the prosecutor on Tuesday.

Daniel Kholodny, the former technical director of Navalny’s YouTube channel, was sentenced to eight years in August, in the same extremism case.

Navalny has been imprisoned in Russia since returning to the country in January 2021, on charges of violating terms of probation related to a historic fraud case, which he dismisses as politically targeted.

In August 2020, he was taken from Russia to Germany after being poisoned with the Soviet-era nerve agent Novichok. Navalny arrived in a coma to a Berlin hospital, following a medical evacuation flight from the Siberian city of Omsk.

Moscow’s clampdown on Navalny precedes the war in Ukraine, but the Kremlin has ramped up measures against internal opposition and free speech since launching its invasion.