Allies of jailed Russian opposition politician Alexei Navalny were facing possible fines and prison time on Friday in court cases that ramp up pressure on Kremlin critics ahead of weekend protests.

Navalny's allies are planning to hold demonstrations on Saturday 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.

Several close Navalny associates, including prominent activist Lyubov Sobol and his spokeswoman Kira Yarmysh, were detained late on Thursday for calling on Russians to join the demonstrations and face hefty fines and short jail stints.

Sobol, 33, is accused of repeatedly violating legislation on public gatherings, which carries a maximum fine of up to 300,000 rubles ($4,000), her lawyer Vladimir Voronin told AFP.

Sobol, who has a small child, in not likely to be given jail time Friday, Voronin said.

But he cautioned that if she is found guilty, authorities could later open a criminal probe against her.

Yarmysh, a 31-year-old Navalny spokeswoman 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 told AFP.

Alexei Navalny returned to Russia from Germany last week after recovering from being poisoned with the Novichok nerve agent
Alexei Navalny returned to Russia from Germany last week after recovering from being poisoned with the Novichok nerve agent AFP / Kirill KUDRYAVTSEV

Ahead of Yarmysh's arrest, she recorded a video urging a large turnout on Saturday.

"Our future literally depends on the number of people who take to the streets," she said.

A number of Navalny allies were also detained in the regions on Thursday and Friday, including the coordinator for the opposition politician's offices in the Far Eastern city of Vladivostok.

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 week Navalny, 44, 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 his arrest he released an investigation into an opulent Black Sea property allegedly owned by Putin.

The two-hour video report had been viewed more than 53 million times since its release on Tuesday, becoming the Kremlin critic's most-watched YouTube investigation.