A Russian court is expected to issue a verdict on Friday in a trial of Aleksei A. Navalny, the jailed opposition leader, on charges of supporting “extremism,” a ruling that could extend his time in prison by up to two decades.

Mr. Navalny, whose anticorruption investigations criticizing the Kremlin drew popular support and infuriated Russia’s top leadership, is already serving a nine-year sentence in a maximum-security penal colony in Melekhovo, 150 miles east of Moscow.

The verdict comes amid intensified suppression of dissent in Russia, which has banned criticism of its war in Ukraine, stepped up its jailing of opposition voices and shuttered liberal news media outlets.

In the case being decided on Friday, Mr. Navalny, 47, is charged with promoting terrorism, funding extremism and rehabilitating Nazism. Prosecutors have called for him to serve an additional 20 years in prison on top of his conviction in March on fraud charges, a case that rights groups said was politically motivated.

Acquittals are extremely rare in Russian courts, especially against opposition figures accused of offenses against state security. Mr. Navalny and Western rights groups have denounced the charges against him as an attempt to silence dissent against President Vladimir V. Putin.

“The sentence will be a long one,” Mr. Navalny said in a statement released by his organization on the Telegram app on Thursday before the expected verdict. “Think about why such a demonstratively huge sentence is needed. Its main purpose is to intimidate. You, not me. I will even say this: you personally, reading these lines,” he added.

The latest charges against Mr. Navalny were laid out in Moscow’s district court in late July, and the trial has been conducted in closed-door hearings at the penal colony where he is being held. His parents tried to attend the trial but were denied entry, according to Mr. Navalny’s organization, which said that his parents have not seen their son for over a year.

Daniel Kholodny, who formerly helped run Mr. Navalny’s YouTube channel, has also been charged in the case with funding and promoting extremism. Prosecutors have asked the court to sentence Mr. Kholodny to 10 years in prison; his verdict is also expected on Friday.

Mr. Navalny nearly died in 2020 after being poisoned with a military-grade nerve agent, an episode that he and Western officials have described as an assassination attempt by the Kremlin. The Russian government has denied involvement.

After receiving medical treatment in Germany, he returned the following year to Russia, where waiting security forces detained him. His group was subsequently outlawed — declared an “extremist” outfit — and the Russian authorities began to crack down even more severely on its activities.

In the first convictions since the group’s banning, two of his associates were sentenced in mid-July to prison terms of seven and a half years and two and a half years for participating in the organization. At least 15 activists who worked with Mr. Navalny face similar charges, according to his spokeswoman, Kira Yarmysh. Many have gone into exile.

Mr. Navalny told a court in late July that he expected to be convicted as well.

“Everyone in Russia knows that someone who seeks justice in court is completely defenseless,” Mr. Navalny told the court, according to his team. “In a country ruled by a criminal, controversial issues are resolved by bargaining, power, bribery, deceit, betrayal and other real-life mechanisms, and not by some kind of law.”