MOSCOW (AP) — A Russian court on Thursday rejected an appeal by opposition leader Alexei Navalny for his release from jail, while authorities detained several of his allies and warned social media companies about promoting more protests after tens of thousands rallied across the country last weekend demanding his freedom.

Appearing in court by video link from jail, Navalny denounced the criminal proceedings against him as part of a government campaign to intimidate the opposition.

“You won't succeed in scaring tens of millions of people who have been robbed by that government,” he said. “Yes, you have the power now to put me in handcuffs, but it's not going to last forever.”

The 44-year-old Navalny, the best-known critic of President Vladimir Putin’s government, was arrested Jan. 17 upon returning from Germany, where he spent five months recovering from nerve-agent poisoning that he blames on the Kremlin. Russian authorities have rejected the accusations.

Navalny was arrested and jailed for 30 days after Russia's prison service alleged he had violated the probation terms of his suspended sentence from a 2014 money-laundering conviction that he has rejected as politically motivated. He also faces accusations in two separate criminal probes.

Before the Moscow Region Court rejected his appeal, defense lawyers argued that while recovering in Germany from the poisoning, Navalny could not register with authorities as required by the terms of his probation. His lawyers also said Navalny's due process rights were repeatedly violated during his arrest.

Navalny described his jailing following an earlier hearing held at a police station as a mockery of justice.

“It was demonstrative lawlessness intended to scare me and all others,” he told the Moscow court.

Navalny's supporters are organizing another round of rallies for Sunday. Police on Wednesday searched Navalny’s apartment, a rented accommodation where his wife, Yulia, has been living and the residences of several of his associates and supporters.

Also detained were Navalny's brother, Oleg; his top ally, Lyubov Sobol; Oleg Stepanov, head of Navalny’s Moscow office; Dr. Anastasia Vasilyeva from the Navalny-backed Alliance of Doctors; and Maria Alyokhina from the Pussy Riot punk collective. They were detained for 48 hours as part of a criminal probe into alleged violations of coronavirus regulations during Saturday's protests.

Putin spokesman Dmitry Peskov said the searches and detentions were a legitimate part of police efforts to investigate the alleged violations during the events.

“Law enforcement agencies are doing their job,” Peskov said on a conference call with reporters. “There were numerous violations of Russian laws, and law enforcement agencies are at work.”

Demonstrations calling for Navalny’s release took place in more than 100 Russian cities Saturday, in a strong show of rising anger toward the Kremlin. Nearly 4,000 people were reportedly detained at those protests and some were given fines and jail terms.

At Thursday’s court hearing, Navalny thanked his supporters and said, “They are the last barrier preventing our country from sliding into the degradation.”

In a later post on Instagram, he urged the Russians to abandon their fear and take to the streets to pressure Putin and his entourage.

“Come out and don't be afraid of anything,” Navalny said. “No one wants to live in the country where lawlessness and corruption run amok. The majority is on our side, let's wake them up.”

Moscow police said 267 people were convicted of violations during the weekend demonstrations in the capital, and 110 of them were given short jail terms. It issued a notice to the public not to join Sunday's protests, warning that officers would act resolutely to disperse unsanctioned rallies and bring participants to justice.

Also Thursday, Russian prosecutors issued warnings to Facebook, Google, Twitter, TikTok and Russian social networks, demanding that they block calls for more protests.

“The state doesn't want the social networks to become a platform for promoting such illegal actions,” Peskov said.

Asked if a refusal to remove such content could prompt Russian authorities to block the platforms, Peskov said it would be up to relevant government agencies to consider a response.

“All pros and cons will be weighed and, if necessary, measures envisaged by the law will be taken,” he said.

Earlier this week, Russian state communications watchdog Roskomnadzor said it would fine Facebook, Instagram, Twitter, TikTok, YouTube and two Russian social networks for their failure to block calls on minors to join Saturday's protests.

A spokesperson for Facebook responded to the AP's request for comment by saying that "there are times when we push back on government requests to remove content which doesn’t break our rules and is clear political speech.”

"We took this step in Russia to allow for speech on last week’s protests,” the spokesperson added in a statement to AP.

Google and TikTok haven’t responded to requests for comment about the Russian authorities’ action and Twitter refused to comment to The Associated Press on Thursday.

Russia’s Investigative Committee said it opened a criminal investigation of Navalny's top strategist, Leonid Volkov, accusing him of encouraging minors to participate in unauthorized rallies. Volkov, who currently stays abroad, rejected the charges.

“The streets must speak now. There is nothing else left,” Volkov tweeted after Navalny's appeal for his release was rejected, repeating the call for Russians to turn out in force Sunday.

In a challenge to Putin two days after Navalny's arrest, his organization released an extensive video report on a palatial seaside compound allegedly built for the president and equipped with fancy amenities, such as a mysterious “aqua-discotheque” that has drawn a stream of sarcastic jokes on the web. The video has been viewed over 99 million times, further stoking discontent.

“If just 2% of the audience will take to the streets, it will be enough to bring all visitors of the 'aqua-discotheque' to their senses,” Navalny said on Instagram.

Navalny fell into a coma Aug. 20 while on a domestic flight from Siberia to Moscow. He was transferred to a Berlin hospital two days later. Labs in Germany, France and Sweden, and tests by the Organization for the Prohibition of Chemical Weapons, established that he was exposed to the Soviet-era Novichok nerve agent.

Russian authorities have refused to open a full-fledged criminal inquiry, citing a lack of evidence that he was poisoned.

Navalny’s arrest and the harsh police actions at the protests have brought wide criticism from the West and calls for his release.

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Associated Press business writer Kelvin Chan in London contributed.