Moscow (CNN) Russian opposition leader Alexey Navalny has said one of his employees was "kidnapped" from his Moscow apartment and sent to a remote Arctic military base, amid a year-end crackdown ahead of Russian holidays.

Navalny, President Vladimir Putin's most vocal opponent, said Ruslan Shaveddinov, a staffer at his Anti-Corruption Foundation (FBK), was taken to Novaya Zemlya island in the Arctic Ocean, "in less than a day."

"I don't know what our Ruslan Shaveddinov did to annoy the authorities this much but it seems that Putin personally came up with a plan to isolate him," Navalny wrote in a blog post Tuesday.

"[First] they blocked his SIM card," he said. Immediately after, Russia's domestic intelligence agency, the Federal Security Service (FSB), broke into Shaveddinov's apartment in the capital.

The Head of Moscow military enlistment office Maksim Loktev told Russian state-news agency TASS that Shaveddinov "for a long time evaded conscription service and his enlistment is legal."

In a follow-up post on Wednesday, Navalny said Shaveddinov was a "political prisoner" who despite being denied a phone was able to contact his colleagues in Moscow, asking them to pass on his medical records that allegedly show he is exempt from military service.

Raid on Navalny office

"[Shaveddinov] is not locked up, but there is an assigned warden who is with him literally every second, so when Ruslan sleeps, eats or goes to the toilet, this person is nearby and watches him," the post continued.

According to Navalny, Shaveddinov's lawyers are filing a lawsuit claiming that, among other things, abuse of power, kidnapping and illegal confinement took place.

Putin's spokesperson Dmitry Peskov said in a conference call with reporters Wednesday that the Kremlin was aware of the reports but declined to give any specifics.

Russian opposition figure Alexei Navalny (center) says his Anti-Corruption Foundation employee Ruslan Shaveddinov (right) is a political prisoner.

"I don't know if he was evading conscription service. If he evaded the draft, he violated the law of the Russian Federation, so I don't think that this casts a shadow over anything," Peskov said.

Navalny himself said Thursday a raid took place on his FBK foundation, but said he had not been detained. His spokesperson said earlier he had been held.

"There are no detainees" in the raid, Navalny said on Twitter. "They [authorities] simply dragged me out of the office."

The anti-corruption campaigner speculated the raid had been timed to disrupt a live internet broadcast, and that authorities had seized his foundation's equipment.

Navalny said he had official information the raid had been carried out over FBK's refusal to take down a video investigation into the purported wealth of Russian Prime Minister Dmitry Medvedev.

That video has been viewed on YouTube more than 32 million times; the Kremlin has dismissed Navalny's allegations.

'Sovereign' internet

On Wednesday a Moscow court announced that the pioneer of the Russian internet, Alexei Soldatov, was placed under house arrest on December 13.

Moscow's Tverskoi district court said in a posting online that Soldatov's house arrest had been ordered on suspicion of fraud. His son, Andrei Soldatov, confirmed his detention to CNN.

Andrei Soldatov, an author and expert on Russia's security services, said he believed the case was connected to recent Russian government efforts to create a "sovereign" internet capable of operating independently of the world wide web, but did not elaborate further.

Earlier this year, Putin has signed into law new measures that would enable the creation of an independent national network. The new measures empower the Russian government to cut off external traffic exchange, creating a purely Russian web.

Russia already has laws on the books that allow authorities to jail individuals who insult government officials online or spread what it calls fake news. Advocates of internet freedom worry that Russia's sovereign internet law creates sweeping new powers to monitor and censor content passing in and out of the country.