Russian police have detained close aides of the jailed Russian opposition politician Alexei Navalny, whose fans have flooded social media with calls to rally in support of him on Saturday.
Moscow police say any unauthorised demonstrations and provocations will be “immediately suppressed”.
More than 55m people have watched his video about President Vladimir Putin’s alleged luxury Black Sea palace.
The Kremlin denies the property belongs to the president.
Mr Navalny was arrested last Sunday after he flew back to Moscow from Berlin, where he had been recovering from a near-fatal nerve agent attack in Russia last August.
On his return, he 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 Thursday 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. Mr Navalny’s Anti-Corruption Foundation (FBK) has drawn millions of followers on social media, through slickly produced videos alleging large-scale official corruption.
He has long denounced Mr Putin’s administration as “feudal” and full of “crooks and thieves”.
In the “Putin’s palace” video Mr Navalny alleges that rich businessmen close to Mr Putin paid for a sumptuous 17,691sq m (190,424sq ft) palace for him at Gelendzhik, by the Black Sea.
It is alleged to have a casino, a theatre and many other comforts, including a vineyard and tea house in the sprawling grounds. The Kremlin dismissed the YouTube video as a “pseudo-investigation” aimed at earning money for Mr Navalny.
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.
Facebook pages of Navalny and Khodorkovsky groups and supporters have been suspended. Even small accounts like the Free Russia Forum’s on Instagram have been blocked after thousands of fake complaints from new Kremlin bot accounts.
— Garry Kasparov (@Kasparov63) January 21, 2021
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In a phone call to President Putin on Friday, EU Council President Charles Michel voiced “grave concern” about the jailing of Mr Navalny.
Mr Michel said the EU was “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”.
In October, the EU imposed sanctions on six top Russian officials and a Russian chemical weapons research centre over the Novichok poisoning of Mr Navalny.
The Kremlin retaliated with tit-for-tat sanctions, denying any role in the attack and rejecting the expert finding that the Russian nerve agent had been used.
The social media app TikTok has a flood of videos from Russians promoting the protests planned for Saturday. The messages about Mr Navalny have been going viral for several days.
A well-known Russian TikTok user, Slava Varfolomeyev, told BBC Russian: “I go on TikTok and find that every third video is about ‘Putin’s palace’, the detention of Navalny and the 23 January rally!”
He said that on Thursday “this swelled to a maximum: practically seven out of every 10 videos were on that topic [Navalny]”. TikTok’s popularity is based on short-form videos.
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, by TikTok or other social media outlets, would bring fines of up to 4m roubles (£39,000; $54,000).
There was also a warning from the Moscow ombudsman for children’s rights, Olga Yaroslavskaya, who said in a Facebook post that “children don’t understand the ways people are manipulated online, don’t understand how skilfully political technologists exploit their developing minds”.
A report by TikTok for October, cited by BBC Russian, reveals that children and young adults under 24 form 32% of TikTok users in Russia. A quarter are aged 25 to 34, and another 20% are 34 to 44. About 25 million people go on TikTok at least once a month in Russia.