Russian police have detained close aides of the jailed Russian opposition politician Alexei Navalny who was arrested last Sunday, 17 January.
His fans have flooded social media with calls to rally in support of him.
According to The Guardian, Moscow police say any unauthorised demonstrations and provocations will be immediately suppressed.
More than 55m people have watched Mr Navalny’s YouTube video about President Vladimir Putin’s alleged luxury Black Sea palace, denied by the Kremlin.
After recovering in Berlin from a near-fatal nerve agent attack in Russia last August, he flew back to Moscow but was immediately taken into custody and found guilty of violating parole conditions. He says it is a trumped-up case designed to silence him.
Among those detained in Moscow on Friday, 22 January, were his spokeswoman, Kira Yarmysh and one of his lawyers, Lyubov Sobol. They face fines or short jail terms.
Ms Sobol, who has a young child, was later released but Ms Yarmysh has now been jailed for nine days.
Prominent Navalny activists are also being held in the cities of Vladivostok, Novosibirsk and Krasnodar.
Unauthorised rallies are being planned in cities across Russia for Saturday, 23 January. Mr Navalny’s Anti-Corruption Foundation (FBK) has drawn millions of followers on social media, through slickly produced videos alleging large-scale official corruption.
Prosecutors have warned people against protesting in support of Mr Navalny on Saturday. Russia’s education ministry has told parents not to allow their children to attend.
Some Russian celebrities in the arts and sports have pledged support for Mr Navalny, they include ice hockey star Artemi Panarin.
Former world chess champion Garry Kasparov, now a leading anti-Putin activist based in the US tweeted that pro-Navalny posts were being widely blocked in Russia.
The social media app TikTok was flooded with videos from Russians promoting the protests. The messages about Mr Navalny have been going viral for several days.
On Wednesday Russia’s official media watchdog, Roskomnadzor, demanded that TikTok take down any information encouraging minors to act illegally. It warned that failure to do so, TikTok or other social media outlets, would bring fines of up to 4m roubles (£39,000; $54,000).