Alexei Navalny, the Russian opposition leader, has stated that he faces additional criminal indictments that might add 15 years to his current nine-year prison sentence.

Mr. Navalny reported that an investigator visited him in prison to inform him that the authorities had begun a fresh investigation against him for "creating an extremist group to fan hatred against officials and oligarchs" and attempting to organize unofficial rallies.

In January 2021, upon his return from Germany, where he had been recovering from nerve-agent poisoning he blamed on the Kremlin, he was detained for the first time.

He received a two-and-a-half-year sentence for a parole violation before receiving a nine-year sentence for fraud and contempt of court.

Mr. Navalny, arguably the most persistent political opponent of Russian President Vladimir Putin, condemned the action as politically motivated in an effort by the government to put him behind bars for as long as possible.

Despite the recent claims, he remained optimistic, stating that he could "thank Putin" for preventing nuclear war by keeping him locked up.

Mr. Navalny's tweet was as follows: "What do I know anyway? Perhaps Putin does not dislike me; maybe he secretly adores me. Therefore, he wishes for me to be concealed in an underground bunker, guarded by trustworthy individuals like himself.

"How else can I explain that not even eight days have elapsed since the start of my 9-year high-security term, and the investigator has already charged me with a new case?

"As it turns out, I founded an extremist organization to stir hatred towards government officials and billionaires. And when they incarcerated me, I dared to be dissatisfied (stupidly) and called for rallies.

"My sentence is intended to be increased by 15 years for this offense.

"That's another fifteen years in a secure, steady bunker where I'll be shielded from the surprises and difficulties of your "freedom."

"Where the streets are so filthy that ordinary citizens, not FSO and FSIN officers, are walking on them.

"My parents, who reside in a tiny military town, came to visit. Therefore, we naturally joked that they would receive one of the first missiles if Putin initiated the nuclear war.

"Who will bomb a prison amid a swamp? I'm having the time of my life.

"Therefore, when the concrete begins to melt, I will merely observe a particularly lovely sunset from the prison yard. Thank you, Vladimir Putin."

The increased term comes after a yearlong Kremlin assault on Mr. Navalny's followers, including other opposition activists and independent journalists, in which officials appeared keen to crush criticism.

After being labeled an extremist organization, his close friends faced criminal charges and fled the country. His group's political infrastructure was destroyed by an anti-corruption foundation and a statewide network of regional offices.