(MOSCOW) — The Russian opposition leader Alexey Navalny has released a message from jail, the first since a Moscow court sentenced him this week to two years and eight months in a prison camp.

In the post published on his Instagram account, via his lawyers, Navalny called on people not to be afraid, telling them he felt free.

“I have, of course, many times heard this phrase: ‘It’s impossible to take freedom from someone who is free inside,"” Navalny wrote. “And now honestly I’m asking, ‘Well, does it work?’”

He continued, “Believe me, it works. The iron doors have slammed behind me with a deafening clang, but I feel myself a free person.”

A judge on Tuesday sentenced Navalny to prison for violating his probation for a 2014 embezzlement sentence, previously found to be unlawful by the European Court of Human Rights.

It has triggered an international outcry from European countries and the United States, which have demanded Navalny’s immediate release. The sentence was condemned as an effort by the Kremlin to neutralize Navalny, President Vladimir Putin’s most difficult critic, after a nerve agent poisoning last summer failed to kill him.

In his post, Navalny said his jailing was Putin’s “revenge” for him daring to return after the assassination attempt and that it exposes the weaknesses of his rule.

“What strength do they have if they have to secretly poison people and illegally jail them?” Navalny wrote.

“They can hold onto power, using it for their personal enrichment, only by relying on our fear,” he said. “On the other hand we, having overcome our fear, can free our Motherland from this little band of occupier-crooks. And we will do that. Truth is on our side. Stay free.”

Since Navalny’s return to Russia, authorities have sought to smother protests that have flared up after his detention, unleashing an exceptional crackdown.

Over the past two weeks, police have detained thousands of people across Russia, including over 6,000 this week alone, according to OVD-Info, an activist group that tracks arrests.

The majority have been released, but hundreds have been sentenced to short administrative detentions in jails from around 10 days to a month.

So many people have been detained in Moscow during the protests that the city’s jails and police cells have overflowed.

Since Sunday, videos have appeared of people appealing for help from police vans, saying they were being kept there because there was no space in jails.

Riot police again detained over a thousand people during protests on Tuesday after Navalny’s sentence, according to OVD-Info, sending another wave of detainees into the city’s packed jails.

With Moscow’s jails full, police on Wednesday night started bringing detainees to an immigrant detention center in the village of Sakharovo outside Moscow, but it too was soon overwhelmed.

Photos posted from inside and provided to ABC News by one of those detained there showed around 25 people packed into a grim cell designed for four or five. In another video, people lie on bare steel cots without mattresses, a filthy open toilet in the room.

Dmitry Ivanov, a student, detained during Tuesday night’s protest and now held in the detention center, told ABC News people were kept waiting in buses before being processed for as much as 24 hours.

“For that time they don’t feed people, they don’t have the chance to sleep, and there’s also no toilet in the bus,” he wrote in a message via Telegram.

Ivanov said people were later moved to cells in smaller groups. He was now with three others, he said.

There have been allegations elsewhere of instances of violence. Alyona Kitayeva, a 21-year-old volunteer working with one of Navalny’s senior team members, told the television channel TV Rain that four police officers had put a plastic bag over her head while she was detained to try to force her to unlock her phone.

“They put it over my head and started to lightly suffocate me,” she told the channel.

The images of the overcrowded cells circulated widely on Russian social media on Thursday, forcing the Kremlin’s spokesman, Dmitry Peskov, to respond.

“There are more arrests than these detention centers can process,” Peskov told reporters. “In the first place, we need to talk about how these are people who took part in illegal protests. There is no repression.”

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