LONDON -- Russian President Vladimir Putin's most prominent opponent could be sentenced Friday to an additional two decades behind bars on extremism charges.

A Russian judge is set to deliver a verdict in the closed-door trial against Alexey Navalny at a courtroom inside the maximum-security prison camp in Melekhovo, about 145 miles east of Moscow, where the Russian opposition leader is already serving 11 1/2 years. Russian prosecutors have requested a 20-year prison sentence for the latest charges, which stem from Navalny's pro-democracy campaigns against Putin's regime.

If the judge finds Navalny guilty, it will be his fifth criminal conviction. All of the charges have been widely viewed as a politically motivated strategy by the Kremlin to silence its fiercest critic.

The 47-year-old lawyer-turned-politician has been in jail since 2021, upon returning to Russia after recovering in Germany from nerve agent poisoning that he blamed on the Kremlin. In 2022, a Russian judge added another nine years to Navalny's sentence of 2 1/2 years on embezzlement and other charges.

Earlier this year, Navalny's team sounded the alarm over his deteriorating health while in solitary confinement, saying he has not received any treatment. They said he has been repeatedly put in solitary confinement for two-week stints for months.

On the eve of Friday's verdict, Navalny said in a social media statement from behind bars that he expects a "Stalinist" sentence of about 18 years.

"When the figure is announced, please show solidarity with me and other political prisoners by thinking for a minute why such an exemplary huge term is necessary," Navalny wrote in the social media post on Thursday. "Its main purpose is to intimidate. You, not me. I'll even say this: you personally, who are reading these words."

In closing statements during his last hearing on July 20, Navalny condemned Russia's ongoing war in neighboring 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.