(MOSCOW) — Russian authorities have moved to designate the organizations of opposition leader Alexey Navalny as “extremist groups” in a step that effectively would outlaw his political movement.

The move is the most serious attack so far by authorities on Navalny’s movement as the Kremlin seeks to break the opposition fomented by its fiercest critic, who was sent to in a prison camp for 2 1/2 years in February.

Russia’s decision comes as doctors supporting Navalny have warned the state of his health is becoming dangerous in prison, where he’s been on a hunger strike for more than two weeks and has accused authorities of denying him medical care.

Russia’s general prosecutor’s office on Friday released a statement saying it had filed a request seeking to have Navalny’s Anti-Corruption Fund as well as his regional campaign branches declared “extremist” under legislation normally used for terrorist groups and violent religious sects.

The prosecutor’s office said it was filing the request on the grounds that Navalny’s groups were “creating conditions for changing the foundations of the constitutional order,” including supposedly through foreign-backed revolution.

Russia in recent years has enacted draconian legislation, nominally to help thwart terror groups, but the measures increasingly are being wielded against critics of President Vladimir Putin. If declared “extremist,” Navalny’s organizations would be banned — anyone deemed to be participating in or aiding them could face lengthy prison sentences.

“Well there we are. They have decided to steamroll the FBK and the campaign headquarters,” Ivan Zhdanov, the Anti-Corruption Fund’s director, wrote on Twitter. “We won’t surrender.”

The Anti-Corruption Fund, known by the initials FBK, publishes investigations revealing the allegedly ill-gotten wealth of Putin and other powerful Russians. The FBK, along with regional branch offices, helps organize peaceful protests against corruption and calls for an end to Putin’s rule. But the groups don’t advocate for violence or overthrowing the state by force.

The Anti-Corruption Fund this week published a new video investigation unveiling what it said was a secret residence for Putin in northwest Russia, complete with an elaborate spa complex.

Leonid Volkov, a top lieutenant of Navalny, said the announcement on Friday meant the Kremlin had still not decided whether to go through with outlawing the group, telling people “don’t keep quiet.”

Authorities have kept up intense pressure on Navalny’s movement since he returned to Russia in January, having recovered from his near fatal poisoning with a nerve agent last summer. His arrest caused thousands to protest, but Navalny’s allies were forced to call off street demonstrations in February in the face of an intense police crackdown.

Concerns have been mounting over Navalny’s health in prison, where, in addition to the hunger strike, he said he’s been refused proper treatment for back pain so severe it limits his walking. And just last week, Navalny was moved to the prison’s medical ward suffering from a respiratory illness and a high temperature.

On Friday, doctors supporting Navalny wrote an open letter to the head of Russia’s prison service pleading for negotiations with prison doctors to agree on a treatment plan, saying Navalny’s worsening condition could be life-threatening.

“We express extreme concern about his state, which is approaching critical,” the doctors, some of who are activists, wrote. The doctors wrote in the letter that medical tests show Navalny is suffering renal impairment that could lead to serious problems his circulatory system “up to a heart attack.”

Navalny’s wife and mother said they visited him this week and were alarmed by how weak he was.

“Aleksey, as always, keeps his spirit. He talks just as cheerfully, but quietly. He coughs badly, breathes with difficulty,” his mother, Lyudmila, wrote in an Instagram post.

Navalny said in a message on Friday that prison authorities were threatening to start force-feeding him if he didn’t feed himself. In a message posted to his Instagram account by his team, Navalny wrote he would refuse and that he was demanding to be examined by his own doctor.

“My head is spinning heavily,” Navalny said, “but I’m still going for now because I feel your support. Thank you!”

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