Russian authorities ramped up pressure on the opposition Wednesday, searching the apartments and offices of jailed Kremlin critic Alexei Navalny ahead of new protests called for the weekend to demand his release.

Ivan Zhdanov, the head of Navalny's FBK Anti-Corruption Foundation, said police were searching flats linked to Navalny and the foundation's offices for alleged violations of coronavirus restrictions.

Russia's Interior Ministry said Wednesday it had launched a criminal probe into the violation of sanitary and epidemiological measures during a Moscow protest that saw thousands rally in support of Navalny on Saturday.

The ministry's spokeswoman Irina Volk said the organisers and participants of the rally "created a threat of the spread of the novel coronavirus infection".

Zhdanov said on Twitter that Navalny's wife Yulia was at one of the apartments and posted a video from inside where loud hammering could be heard outside the door.

"They are not letting in my lawyer. They broke my door in," Yulia Navalnaya yelled to journalists from out of her apartment window, an AFP journalist reported.

-- 'Violation of law' --

Standing outside the apartment, her lawyer Veronika Polyakova told AFP that the police were not letting her in, which is a "violation of the law".

Searches were carried out at the homes of several other Navalny allies, Zhdanov added.

Zhdanov posted a screenshot from a security camera at the office of FBK, showing several masked men there.

The foundation is best known for its investigations into the wealth of Russia's political elite.

Its most recent report suggested President Vladimir Putin was given an opulent property on the Black Sea Coast costing over $1.5 billion as a gift.

The investigation was released days after Navalny was arrested on his return on January 17 from Germany, where he spent months recovering from a nerve agent poisoning he blames on the Kremlin.

Navalny's Anti-Corruption Foundation says police searched flats linked to the opposition figure and the foundation's offices / © AFP/File

Tens of thousands of people demonstrated in cities across Russia on Saturday calling for Navalny's release.

According to independent monitors, more than 3,900 people were detained at the demonstrations while authorities launched a series of criminal probes over the protests.

In Washington, US Secretary of State Antony Blinken voiced "deep concern" about Navalny's safety.

"It remains striking to me how concerned, and maybe even scared, the Russian government seems to be of one man -- Mr. Navalny," Blinken told a news conference on his first full day on the job.

"The larger point is that his voice is the voice of many, many, many Russians, and it should be heard, not muzzled,' Blinken said.

-- Attack on tech firms --

Blinken's remarks come a day after the Group of Seven industrial democracies jointly called for the "immediate and unconditional" release of Navalny.

Russia's Foreign Ministry on Wednesday called the statement "gross interference" in the country's domestic affairs and an "openly unfriendly step".

Authorities also upped pressure on online platforms for failing to delete posts calling for minors to join the unsanctioned rallies.

Protests in Russia are banned if they are not approved by the authorities, as are calls for people under 18 to join demonstrations.

Russia's media watchdog Roskomnadzor said Wednesday that platforms including Facebook, Instagram, Twitter, TikTok and YouTube, could be fined for failing to delete posts that called for minors attend to the rallies.

It added that the fines would range from 800,000 rubles ($10,500) to 4 million rubles ($53,000).

Putin for his part warned of the growing influence of large technology companies, which he said are "competing" with states.

The foreign ministry in turn accused the US embassy of distributing fake and provocative content and said it handed a note of protest "with a warning that the Russian side reserves the right to retaliate".