MOSCOW — Russian opposition leader Alexei Navalny wasted no time showing why the Kremlin finds him such a threat: From behind bars in a coronavirus isolation cell, he released a bombshell video accusing President Vladimir Putin of colossal corruption.

The YouTube video — released Tuesday less than 48 hours after Navalny returned to Russia in a direct challenge to Putin and his security services — crossed all Putin’s red lines.

It also underscores that Navalny and his supporters appear united and organized five months after the opposition leader suffered a near-fatal nerve agent poisoning he claims was ordered by Putin. The Kremlin denies the claim.

Protests by Navalny’s backers are planned across Russia on Saturday.

But videos and social media — anchored by his network of 40 offices across Russia — remain the core of Navalny’s opposition power, pointing out alleged abuses and indulgences by Russia’s leaders under Putin. Navalny’s election ­efforts back candidates with the best chances of ousting members of Putin’s party.

This time, the video pointed directly at Putin. It included a photo of teenager Elizaveta Krivonogikh, who it claimed was the secret daughter Putin fathered with a lover. Navalny’s video also published an architectural plan and drone footage of a gigantic palace near Gelendzhik on the Black Sea, including a cellar winery, an indoor ice rink and a casino. The video alleged it was built for Putin using a complex “slush fund.”

“Putin’s palace. History of world’s largest bribe,” viewed more than 25 million times, also named former gymnast Alina Kabaeva as another woman who allegedly received benefits such as apartments for herself and her family through Putin. Neither Kabaeva nor Krivonogikh has commented publicly.

Putin’s spokesman Dmitry Peskov said the video was “a con” and “pure nonsense,” denying that the palace was related to Putin, but he gave no details on who they say is the owner. Peskov also has rejected a November report by a Russian outlet, Proekt Media, that Krivonogikh was related to Putin as “unfounded and unconvincing.”

Releasing such an explosive video while in the clutches of a judicial system notorious for its politicized decisions is a risk for Navalny.

He was detained Sunday upon his return from Berlin and could face at least 2 ½ years on charges he violated a suspended sentence. He also is looking at possible lengthy prison terms in two other criminal cases for alleged fraud and embezzlement. Navalny says the charges are political.

After surviving poisoning by a Novichok group nerve agent in August during a trip to Siberia, he directly confronted the domestic intelligence service, or FSB. Navalny phoned a member of the hit team that investigative reporting group Bellingcat linked to the crime, tricking him into revealing details of the poisoning last month.

Navalny, 44, is seen by the Kremlin and securocrats as an enemy of the state, according to political analyst Tatiana Stanovaya of the R. Politik analytical firm. Bringing down Navalny is now seen as “a matter of honor” by the FSB, she said.

“They want to see Navalny destroyed in prison, physically and mentally weak and vulnerable,” she said. “They don’t care about protests or elections. They would like to see the West increase sanctions because it gives them more arguments to insist on this harsh line in Russian policy.”

Leonid Volkov, head of Navalny’s network of offices, believes Navalny’s life remains in danger.

“The situation is very dangerous indeed because, technically and practically, Alexei Navalny is now in the custody of the very people who tried to poison him,” said Volkov, who is organizing the protests planned for Saturday. The Kremlin denies any link to the poisoning.

“The Russian Orwellian reality actually demands a lot of bravery now,” he added.

Navalny’s poisoning sent the message that serious opposition rivals to Putin would not be tolerated. Since then, authorities have passed a raft of laws aimed at Navalny and other dissidents, making it harder to protest, oppose the regime, or expose corruption by security officials and others.

“There were some unwritten rules about what you could do and not do to be on the safe side, but this has changed,” Volkov said. “So what really makes practical sense is to do what you have to do and that’s it.”

Navalny was still in intensive care in Berlin after his poisoning when he conceived the video as what he sees as the definitive account of Putin’s corrupt regime. He called it a “psychological portrait” of a man he says was an unexceptional former KGB agent who became obsessed with amassing unlimited wealth and staying in power.

The video’s visualizations of palace interiors with ornate Italian furniture are based on drawings that Navalny said were leaked by a contractor “stunned and enraged by the luxurious decorations and the insane prices of the furnishings.”

There were details: a wine-
production cellar with classical musical piped in 24 hours a day to help the vintages mature; a ­hookah room with a pole-dancing stage; a tunnel to a sea lookout; an underground ice hockey rink; a two-story theater; a casino; and an “aquadiscoteque.” There were custom-made sofas for $27,000, and tables costing up to $56,000.

Businessman Sergei Kolesni­kov revealed some details in 2010, saying he had been involved in the project to build a residence for Putin. Photographs of the lavish interiors were posted online by workers in 2011.

On Navalny’s first day in Moscow’s Matrosskaya Tishina detention center on Tuesday, Vladimir Ashurkov, London-based executive director of Navalny’s Anti-Corruption Foundation, called on Western governments to issue sanctions against a list of eight Russians with links to Putin. The roster includes billionaire Roman Abramovich, owner of the Chelsea soccer club in Britain, and Health Minister Mikhail Murashko, accused of “covering up Alexei’s poisoning and hindering efforts to evacuate him to Germany for medical treatment.”

“People on the shortlist are some of the key architects of the system of corruption, restriction of political freedom and oppression that Putin’s regime has built over years in Russia,” Ashurkov said in an interview.

“It will be a deterrent to other people not on this list,” he added. “The message is that there will be consequences to the support of Putin’s regime and to the oppression that these people bring to Russia.”

Rola Brentlin, spokeswoman for Abramovich, strongly rejected Ashurkov’s claim that Abramovich was responsible for any corruption, oppression or restriction of political freedom in Russia.

There has been no immediate reaction to the sanctions appeal from Western nations.

Ashurkov said Navalny knew he might be jailed, but “will find a way to have his voice heard.” At the same time, Putin’s popularity has been ebbing, real incomes declining and the Kremlin is struggling to ignite a sense of national momentum and pride amid the crippling pandemic.

“People are depressed,” said Dmitry Gudkov, an opposition figure unaffiliated with Navalny, adding that many people were angered by Navalny’s detention but afraid to heed his call for street protests. “Many people are threatened by the regime because people were sent to prison for just participating in actions and meetings.”

“Putin is losing popularity, and law enforcement and the FSB and repression will be the key instruments to sustain power,” Gudkov said. “If you can’t buy loyalty, you can trade on fears. You can intimidate people and threaten people to prevent them from participating in actions. That’s the trend in our country.”

At the end of the video, Navalny urges people to “believe in our strength” and take to the streets.

“All we have to do is stop being patient. Stop wasting your life and your taxes enriching these people,” he said. “Our future is in our hands.”