MOSCOW (AP) — Russian authorities on Thursday detained several allies of opposition leader Alexei Navalny and issued warnings to social media giants after tens of thousands of people took to the streets in over 100 Russian cities last weekend to demand his release.

The detention of Navalny's brother Oleg, his top ally Lyubov Sobol, Dr. Anastasia Vasilyeva from the Navalny-backed Alliance of Doctors and Maria Alyokhina from the Pussy Riot punk collective comes as authorities try to stem another wave of protests set for Sunday.

All four were detained for 48 hours as part of a criminal probe into alleged violations of coronavirus regulations during the weekend's protests.

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

A court in Moscow is to consider his appeal against the arrest later on Thursday.

The overnight detentions came after more than a dozen searches of apartments and offices of Navalny's family, associates and supporters in connection to the probe. Those sites included Navalny’s apartment, where police detained his brother, and a rented apartment where Navalny’s wife, Yulia, has been living.

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

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

Russian prosecutors on Thursday also 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 their refusal to remove such content could prompt Russian authorities to block them, Peskov responded 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, Russia's 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.

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 lives in Germany, rejected the charges.

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.

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.