MOSCOW (AP) — A Russian court on Thursday rejected opposition leader Alexei Navalny's appeal of his arrest while authorities detained several of his allies and issued warnings to social media companies after tens of thousands swarmed the streets in over 100 Russian cities last weekend demanding his release.

Appearing in court by video link from jail, Navalny denounced criminal proceedings against him as part of the government's efforts 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 most well-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 at the request of Russia's penitentiary service, which charged that he had violated the probation terms of his suspended sentence from a 2014 money-laundering conviction that rejected as politically driven. He is also currently facing accusations in two separate criminal probes.

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

Navalny described his jailing following an earlier court hearing quickly 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.

Navalny's brother, Oleg Navalny, his top ally, Lyubov Sobol, Dr. Anastasia Vasilyeva from the Navalny-backed Alliance of Doctors and Maria Alyokhina from the Pussy Riot punk collective were detained for 48 hours as part of a criminal probe into alleged violations of coronavirus regulations during last 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 during a conference call with reporters. “There were numerous violations of Russian laws, and law enforcement agencies are at work.”

Moscow police on Thursday issued a notice to the public not to join protests Sunday, 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.

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

Also Thursday, Russia's Investigative Committee said it opened a criminal probe against 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 was rejected, repeating the call on Russians to turn out in force on 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. It has been viewed over 98 million times, further stoking discontent.

Demonstrations calling for Navalny's release took place in more than 100 cities across the nation last Saturday, a strong show of rising anger toward the Kremlin. Nearly 4,000 people were reported detained at those protests and some were handed fines and jail terms.

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

Navalny fell into a coma while aboard a domestic flight from Siberia to Moscow on Aug. 20. He was transferred from a hospital in Siberia 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 Navalny 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 to this report.