Jailed Russian Opposition politician Alexei Navalny said on Thursday that authorities had opened a new criminal case against him for promoting terrorism and extremism, potentially more than doubling his sentence.

Navalny, the most prominent domestic critic of President Vladimir Putin, is already serving prison terms totalling 11-1/2 years for fraud, contempt of court and parole violations, all of which he rejects as trumped-up charges intended to silence him.

"I am a genius of the underworld. Professor Moriarty is no match for me,” he said sarcastically in a Twitter post, comparing himself to the nemesis of Sherlock Holmes.

"You all thought I had been isolated in prison for two years, but it turns out I was actively committing crimes. Luckily, the Investigative Committee was vigilant and didn't miss a thing.” Navalny said his lawyers estimated he could now face a cumulative sentence of around 30 years.

There was no immediate official confirmation of the new case from the Investigative Committee. Navalny's spokeswoman Kira Yarmysh said the case related to a YouTube channel called Popular Politics that was launched by his allies after he had already been in prison for a year.

Navalny has long been a thorn in Putin's side, campaigning against endemic corruption in Russia in witty and slickly produced videos that drew huge audiences on social media. In 2020, he was poisoned with a nerve agent during a campaigning trip in Siberia, according to analysis conducted by several European medical institutions. The Kremlin denied involvement.

After months of medical treatment in Germany, he was arrested for parole violations when he returned to Russia at the start of 2021. Since Russia's invasion of Ukraine, the Kremlin has accelerated its campaign to quash and silence domestic opposition. Navalny has spoken out against the war, attacking Putin during a court appearance and calling the invasion ”stupid” and“built on lies”.