Navalny said by video link from prison during the hearing that the extremism charges could land him in prison for 30 years. He added that an investigator had told him that he also would face a separate military court trial on terrorism charges that could potentially carry a life sentence.
His top ally said that investigators were trying to link the terrorism charges against Navalny to a bombing that killed a well-known Russian military blogger earlier this month.
[bsa_pro_ad_space id=1]Navalny, 46, who exposed official corruption and organised massive anti-Kremlin protests, was arrested in January 2021 upon returning to Moscow after recuperating in Germany from nerve-agent poisoning that he blamed on the Kremlin.
Also read: EU weighs response to Navalny arrest, protest crackdown
He initially received a 2 -year prison sentence for a parole violation. Last year, he was sentenced to a nine-year term for fraud and contempt of court. He is currently serving time at a maximum-security prison 250 kilometres (150 miles) east of Moscow.
The new charges against Navalny relate to the activities of his anti-corruption foundation and statements by his top associates. His ally, Leonid Volkov, said the accusations retroactively criminalise all the activities of Navalny's foundation since its creation in 2011 and carry a potential punishment of up to 35 years in prison.
Navalny's associate, Ivan Zhdanov, said Wednesday that investigators were revising the charges to link them to a bombing that killed Russian military blogger Vladlen Tatarsky at a St. Petersburg cafe earlier this month.
The authorities described Darya Trepova, a 26-year-old St. Petersburg resident who was seen on video presenting Tatarsky with a statuette moments before the blast, as an active supporter of Navalny. They also accused Zhdanov and Volkov of making repeated calls for subversive activities in Russia.
The new charges come as Russian authorities conduct an intensifying crackdown on dissent amid the fighting in Ukraine, which Navalny has harshly criticised.
Wednesday's hearing at Moscow's Basmanny District Court was held to discuss preparations for Navalny's trial on the extremism charges. Navalny asked for more time to study the 196 case files.
[bsa_pro_ad_space id=1]The judge closed the session minutes after it opened, ruling that it should be held behind closed doors, because the case involved sensitive information.
"It's an attempt to unlawfully restrict my ability to study the materials of the case and prevent anyone from knowing about it," Navalny said before public access to the hearing ended.
The hearing ended with the judge giving Navalny 10 days to study his criminal case. No date for the trial has been set yet.
Navalny, who is President Vladimir Putin's most prominent foe, has rejected the charges as a political vendetta and an attempt by Russian authorities to keep him out of politics for as long as possible.
His associates and supporters have become increasingly worried about his failing health. Earlier this month, they said Navalny had fallen ill with acute stomach pains and suspected that he was being slowly poisoned.
While imprisoned, Navalny has spent months in a tiny one-person cell, also called a "punishment cell," for purported disciplinary violations such as an alleged failure to properly button his prison robe, properly introduce himself to a guard or to wash his face at a specified time.
His supporters have accused prison authorities of failing to provide him with proper medical assistance, using blindingly bright light in his cell and placing him next to a mentally unstable person.
Navalny said Tuesday that he had completed a 15-day stay in the punishment cell and was immediately ordered to spend another 15 days there.