Moscow police have arrested and detained Oleg Navalny, the brother of jailed Russian opposition leader Alexei Navalny, and searched the homes and offices of the Kremlin critic's associates including an apartment where his wife was staying and other properties linked to him, reports said Thursday.

The searches were made after tens of thousands of Navalny supporters took to the streets on Saturday to demand the Kremlin to free him from prison. Alexei Navalny is serving a 30-day detention for alleged parole violations that he denies.

Protests demanding Alexei's release took place in more than 100 cities across Russia at the weekend. Almost 4,000 demonstrators were reportedly arrested during those protests. Alexei's allies plan to hold new demonstrations this Sunday.

Authorities immediately arrested Alexei on his return to Moscow last week. He had been recovering in Germany since he was poisoned last summer. He has not yet returned to his home since the poisoning incident which nearly killed him.

Oleg Navalny was released from detention in 2018 after serving three and a half years for an embezzlement conviction that critics say was intended to put pressure on his brother Alexei, who was given a suspended sentence in the same case.

Police also searched the offices of the Anti-Corruption Foundation, Alexei's allies said. Images on social media showed around 20 masked men waiting to enter the offices.

There was no immediate comment from the authorities regarding the searches. Alexei's associates said the searches were in relation to alleged violations of epidemiological regulations from last week's mass demonstrations in Moscow.

Ivan Zhdanov, chief of Alexei's Anti-Corruption Foundation, said Oleg would remain in police custody for 48 hours on allegations of violating sanitary and epidemiological regulations.