Aides of jailed Russian opposition politician Alexei Navalny face possible fines and prison time as police vow to crack down on opposition protests this weekend.

His allies are planning to hold demonstrations tomorrow in dozens of cities in support of the Kremlin critic, who was arrested and jailed on his return to Russia following a near-fatal poisoning with a nerve agent.

"Attempts to hold unsanctioned public events, as well as any provocative actions on the part of their participants, will be regarded as a threat to public order and immediately suppressed," Moscow police said.

Mr Navalny's associates urged Russians to take to the streets despite official pressure and promised financial help for protesters given fines.

"Putin is doing everything to intimidate you," Ivan Zhdanov, the head of Mr Navalny's Anti-Corruption Foundation, wrote on Facebook.

Several close associates of the opposition politician, including prominent activist Lyubov Sobol and his spokeswoman Kira Yarmysh, were detained last night for calling on Russians to join the demonstrations and face hefty fines and short prison terms.

Ms Sobol is accused of repeatedly violating legislation on public gatherings, which carries a maximum fine of up to 300,000 rubles (€3,284), her lawyer Vladimir Voronin said.

Ms Sobol, who has a small child, is unlikely to be given jail time, Mr Voronin said.

But he cautioned that if the 33-year-old is found guilty, authorities could later open a criminal probe against her.

Ms Yarmysh, a 31-year-old spokeswoman for Mr Navalny who spent the night in jail, is accused of violating legislation on public gatherings and could be detained for 10 days, her lawyer Veronika Polyakova said.

Russian President Vladimir Putin

A number of Mr Navalny's allies were also detained in the regions, including the coordinator for the opposition politician's offices in the eastern city of Vladivostok and a volunteer in the western exclave of Kaliningrad.

Both were jailed for three days.

Prosecutors have warned Russians against taking to the streets during the coronavirus pandemic, while the state communications watchdog cautioned social media platforms including video app TikTok against encouraging minors to participate in the rallies.

Last Sunday Mr Navalny returned to Russia from Germany, where he had been recovering from a poisoning with the Novichok nerve agent in an attack he blamed on Russian security services and President Vladimir Putin.

After the 44-year-old's arrest his team released an investigation into an opulent Black Sea property allegedly owned by Mr Putin.

The two-hour video report has been viewed more than 50 million times since its release on Tuesday, becoming the Kremlin critic's most-watched YouTube investigation.

Mr Putin's spokesman Dmitry Peskov denied that the mansion belonged to the president, calling Mr Navalny's probe "lies."

"Putin has nothing to do with this," Mr Peskov said, adding that the mansion - which the report claimed is being protected by the FSB domestic intelligence agency - might belong to a businessman.

Following Mr Navalny's arrest and latest graft report, many Russians took to social media - including TikTok and dating app Tinder - to voice support and urge a large turnout tomorrow.

Meanwhile, European Council President Charles Michel held a phone call with Mr Putin today and pressed for the release of Mr Navalny.

Mr Michel's office said he told Mr Putin that "the EU is united in its call on Russia to swiftly release Mr Navalny and proceed with the investigation into the assassination attempt on him, in full transparency and without further delay."