Russian authorities on Friday cracked down on supporters of opposition politician Alexei Navalny, handing down short jail terms to allies as police vowed to suppress protests this weekend.

Navalny's supporters are planning to hold demonstrations Saturday in dozens of cities after he was detained on his return to Russia following a near-fatal poisoning with a nerve agent.

The rallies are expected to be a major test of the opposition's ability to mobilise after police said unsanctioned public events will be "immediately suppressed."

In an Instagram post Friday Navalny thanked his supporters and said in jest he didn't plan to harm himself while in detention.

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

"I don't plan on hanging myself from the prison bars," he quipped. "My psychological and emotional state is completely stable".

Navalny's spokeswoman Kira Yarmysh, 31, was sentenced to nine days in jail for violating laws on public gatherings in a hearing that lasted less than five minutes, her lawyer Veronika Polyakova told AFP.

Another activist, Georgy Alburov, said he was jailed for 10 days while key Navalny ally Lyubov Sobol was fined 250,000 rubles (around $3,300).

A number of Navalny associates were detained in the provinces and also received short jail terms.

Despite the increasing pressure, Navalny's aides urged Russians to take to the streets and promised financial help to protesters handed fines on Saturday.

Lyubov Sobol is accused of repeatedly violating legislation on public gatherings
Lyubov Sobol is accused of repeatedly violating legislation on public gatherings AFP / Alexander NEMENOV

In Moscow, demonstrators are being urged to gather in the city centre and then march towards the Kremlin.

Citing the pandemic, Moscow Mayor Sergei Sobyanin said the opposition's calls for demonstrations were "unacceptable" and cautioned that police would "ensure necessary order".

In a Instagram post on Friday, the 44-year-old opposition politician's wife Yulia Navalnaya wrote that she would join the protests in Moscow.

"For myself, for him, for our children, for the values and ideals that we share," she wrote, explaining her decision to join the demonstrations.

Kremlin critic Alexei Navalny urges Russians to stage mass anti-government protests during a court hearing after his arrest on arrival in Moscow from Germany.
Kremlin critic Alexei Navalny urges Russians to stage mass anti-government protests during a court hearing after his arrest on arrival in Moscow from Germany. NAVALNY TEAM'S YOUTUBE PAGE

Many Russians took to social media -- including video sharing app TikTok and even dating app Tinder -- to voice support and urge a large turnout on Saturday.

A hashtag demanding freedom for Navalny was trending on TikTok as Russians posted thousands of videos.

"This is of course something shocking," said political analyst Tatiana Stanovaya, adding that the TikTok "creative frenzy" should be closely watched.

Margarita Simonyan, editor-in-chief of Kremlin-backed broadcaster RT, accused TikTok pitting Russian children against each other.

The communications watchdog has cautioned social media platforms against encouraging minors to participate in the rallies or risk fines of up to four million rubles ($53,000).

Roskomnadzor said Friday the media platforms began removing the content in question.

Navalny last week returned to Russia from Germany, where he had been recovering from a poisoning with the Novichok nerve agent in an attack he blamed on Putin.

After his arrest his team 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.

Putin's spokesman Dmitry Peskov on Friday once again denied the mansion belonged to Putin, calling Navalny's probe "lies."

"Putin has nothing to do with this," Peskov said, adding that the mansion might belong to a businessman.

A number of public figures -- including those who usually steer clear of politics -- have spoken out in Navalny's support.

"This is not about politics but about civil society and justice," she said.