MOSCOW — Russian authorities from the Kremlin down warned Russians on Friday not to join Saturday protests to support jailed opposition leader Alexei Navalny, as police detained key opposition activists and jailed two members of his team. But dozens of prominent social media influencers, celebrities and sports stars expressed horror and anger at President Vladimir Putin's authoritarian turn.

In a letter reportedly written from prison, Navalny suggested his life was at risk and declared that, "just in case," he has no intention of killing himself.

“Right now it’s impossible to further shut up in this situation because once they deal with him, they’ll deal with everyone else who’s critical in the slightest,” said Dmitry Glukhovsky, author of a cult dystopian science fiction trilogy and survival games series, in an interview Friday.

“If you keep shutting up and you keep pretending that everything is normal, you get the feeling that the situation will deteriorate way faster and it will be completely uncontrollable,” said Glukhovsky, whose young fan base includes many gamers. “In every looming and sickening dictatorship, if you do not back the newly arrested person, soon they will come after you.”

The disconnect between the Kremlin and Russian social media influencers underscored Putin’s problems in reaching beyond his aging, conservative base to connect with the young, urban Russians who want to be part of the modern world.

Igor Denisov, the former captain of Russia’s national soccer team, also came out in support of Navalny. His comments showed the depth of alarm in Russia over Navalny’s near-fatal poisoning in August and his imprisonment immediately after he flew home from Germany, where he received treatment.

“I have never been interested in politics, and I will never be interested in politics,” Denisov said. “But this is not about politics.”

He added: “I want to support Alexei Navalny and his family, his wife and his children. Alexei should be free. I do respect him. I wish everybody peace and kindness.”

It is unusual in Russia for sports heroes to speak out politically, but hockey star Artyom Panarin, one of the country’s top players, also posted “Free Navalny” on Instagram.

Russian authorities rounded up members of Navalny’s team ahead of Saturday’s planned rallies. His press secretary, Kira Yarmysh, tweeted late Thursday that police were pounding on her door telling her she would be detained before the Moscow protest rally. She was jailed Friday for nine days.

Georgy Alburov, another member of Navalny’s team and co-author of a viral video, “Putin’s Palace: History of the World’s Biggest Bribe,” was jailed Friday for 10 days. The video, which alleges that a massive luxury palace was built for Putin on the Black Sea, has been viewed more than 57 million times.

Navalny’s team posted a letter Friday on Navalny’s Instagram page that he was said to have written in the Matrosskaya Tishina detention center. In it, he thanked supporters after learning from his lawyer that the video had gone viral. He also suggested that its success could anger the Kremlin and that his life was in danger.

“Therefore, just in case, I declare it is not my plan to hang myself on the window bars, or to open my veins or throat with a sharpened spoon,” Navalny’s letter said. “My psycho-emotional state is completely stable. I know for sure that there are many good people outside my prison and help will come.”

Political analyst Andrei Kolesnikov of the Carnegie Moscow Center predicted that authorities would ignore the mounting pressure to free Navalny as the regime moves to a full-scale authoritarian approach, crushing political opponents and civic activists.

“They will not listen to anyone,” he said. “This is a kind of civil war on civil society by the state.”

Even so, the outpouring of support for Navalny from celebrities and social media influencers, such as popular video blogger Yury Dud, poses a major challenge to the Kremlin, Kolesnikov added.

“The current young generation is more radical than previous generations,” he said. “They want to live in a modernized Russia, not an old-fashioned, traditional Russia. This is a generation that was born under Putin and is still living under Putin. For them, Navalny is a more inspiring person, if they compare Navalny as a patriotically oriented figure who returns to Russia with Putin, who is much older and more old-fashioned.”

Russia’s Interior Ministry warned Friday that tough action would be taken against protesters Saturday. Unsanctioned protests “will be regarded as a threat to public order and will be immediately suppressed,” it said. Opposition social media pages were shut down.

Navalny’s wife, Yulia Navalnaya, said Friday on Instagram that she will demonstrate Saturday “for myself, for him, for our children, for the values and ideals we share,” despite the risk of arrest.

“I will come out for an amazing, very talented politician who is making Russia better despite all odds,” she said. “I’ll go for the guy who tweets funny jokes and does great investigative work. I’ll go for the fearless, courageous man who gets poisoned . . . and never gives up no matter what. I’m going for the man I love very much.”

An opposition lawmaker in Novosibirsk, Sergei Boiko, was arrested. Many other members of Navalny’s team were arrested in cities across Russia. Lyubov Sobol, a prominent member of Navalny’s team, was detained Thursday but released the same night.

Despite the crackdown, however, Russian authorities appeared to be losing control of the narrative amid an outpouring of support for Navalny from Russian celebrities.

“For all those who didn’t know it, or who simply doubted: Formerly it was gangsters who committed mayhem, but now the state does it easily,” said Dud, the video blogger who has more than 8 million YouTube followers. “The biggest risk here is that when lawlessness becomes the norm, very often everyone becomes its victim, including those who once established and enforced this norm.”

Well-known actress Yana Troyanova posted that Russia was being “plundered in a completely insolent way,” referring to the “Putin’s Palace” video. She called on Russians to join the Saturday protests “just to feel that you are a free person.”

“I thought that nothing could surprise me, after assassination attempts, attempts at his life and Yulia’s life, the numerous political prisoners,” she wrote, referring to Navalny and his wife. “But I am shocked after this film. Are we going to be silent after this film? Our country is being robbed.”

A young Russian pop star, Elizaveta Gyrdymova, known as “Monetochka,” posted a song on Instagram in support of Navalny’s freedom. “This is not about politics, but about civil society and justice,” she commented.

Other supporters included rapper Noize MC, popular TV host and blogger Alyona Vodonayeva, actress Varvara Shmykova and news presenter Leonid Parfyonov.

“I thought such evil kings only existed in some very scary fairy tales. But no, this is our reality,” Vodonayeva said, referring to Putin.

The first novel in Glukhovsky’s dystopian science fiction trilogy, “Metro 2033,” set in the Moscow Metro in a post-apocalyptic world, tells a dark story of fascistic leaders who construct a big lie to fool people to keep them trapped underground after a nuclear holocaust. He said he was not a particular Navalny supporter but that it was impossible to ignore the authoritarian turn after what he called “a chain of murderous poisonings,” not only of Navalny but of other Kremlin critics.

“How can you be for the entire state machinery of Russia opposing the only truly independent prominent politician, with first trying to eliminate him physically,” he said in an interview Friday. “How can you be for banning all kinds of civilized political activity in the country, including penalizing even a single [person who] protests, turning Russia definitely into an autocracy, prolonging Putin’s ability to stay in power personally for another decade and a half from now?”

Kremlin spokesman Dmitry Peskov warned that protests are illegal and unacceptable. He said “provocateurs” were behind the protests.

“There can be only one position — the position of the unconditional need to comply with the law and the inadmissibility of organizing illegal actions, and even more so provoking the participation of young people, children and so on in these actions,” he said.

Russia’s telecommunications authority, Roskomnadzor, said Friday that social media platforms TikTok, YouTube, Instagram and Russia’s VKontakte were heeding its demands to take down videos urging “children” to participate in Saturday’s protests.