More than 1,000 people have been detained at protests across Russia in support of jailed Kremlin critic Alexei Navalny.
Supporters of President Vladimir Putin's most prominent opponent fear he soon could die in prison, where he is on hunger strike, and are demanding he be given proper medical care.
Officials say he has been treated as any other convict would be, and have warned the rallies are illegal.
Earlier, Mr Putin warned foreign rivals to tread lightly as he gave a key speech amid deep tensions with the West and arrests of opposition protesters.
Addressing politicians and senior officials in his annual state of the nation address, Mr Putin said anyone "crossing the red line" with Russia could expect a harsh response.
Hailing the country's battle against the coronavirus and development of vaccines, he said Russia needed to tackle climate change and - with parliamentary elections due in September - announced a raft of populist social spending measures.
As he spoke, Russian police were detaining protesters.
Two close aides of Mr Navalny were detained by police in Moscow, while monitors reported police raids on his offices and arrests of his supporters across the country.
Mr Putin unsurprisingly made no mention of Navalny in his speech - he has always refused to use his critic's name - or of any other opposition to his leadership.
He did however hit out at rivals abroad, with Moscow and Western capitals at loggerheads over Navalny, a Russian troop build-up on Ukraine's borders and a series of espionage scandals that resulted in diplomatic expulsions.
Mr Putin said it had become "a new kind of sport" in some foreign capitals to blame Russia "for anything".
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He said Russia wanted good relations with everyone in the international community, but warned of a "harsh" response if that was seen as weakness.
"I hope that no one will think of crossing the red line in relation to Russia. And where it will be - we will determine that ourselves," he said.
Mr Putin backed claims by Belarusian President Alexander Lukashenko that his security services had thwarted an alleged US plot to assassinate him, suggesting senior US officials were involved in a "coup attempt" and accusing the West of pretending "that nothing is happening".
He is due to meet Mr Lukashenko - who has faced down historic protests since a disputed re-election last summer - in Moscow tomorrow, amid speculation of a major announcement on Russia's policy towards its ex-Soviet neighbour and ally.
Mr Putin began his speech by hailing the country's response to the Covid-19 pandemic, describing Russia's development of three vaccines as a "real breakthrough" and saying the country was aiming for herd immunity by autumn.
He vowed that Russia, one of the world's major producers of oil and gas, would do its part to fight climate change, setting a target for the country's emissions to be "less than in the European Union".
Much of his speech was devoted to new social spending, as he looked to shore up support for his deeply unpopular United Russia party ahead of parliamentary elections in September.
Mr Putin's popularity has long been based on his ability to provide stability and better living standards to Russians, but the economy has in recent years been hit by Western sanctions, stagnant oil prices and now the coronavirus pandemic.
"The main thing is to ensure the growth of citizens' real incomes," he said, adding that increases in consumer prices were "eating away" at Russians' incomes.
He announced new lump-sum payments to families and expectant mothers, as well as a 10,000 ruble (€108) payment for all schoolchildren in mid-August.
Navalny's supporters were hoping to steal the president's thunder with a series of mass protests starting from 7pm in cities across the country.
Security forces had issued a warning against taking part in "illegal gatherings" and appeared to be moving quickly to deter protesters.
Police pulled Navalny ally Lyubov Sobol out of a taxi near Navalny's main offices in Moscow and detained her, Ms Sobol's lawyer said.
Spokeswoman Kira Yarmysh said she was also detained at the entrance to the building, while independent monitor OVD-Info said police had conducted searches and detained at least 53 people in 27 cities.
Yesterday, Navalny's doctors were denied access to him, as his lawyers said he was "very weak" and demanded he be transferred to a civilian hospital.
A team of medics has been trying to see the 44-year-old since early this month, after he began refusing food on 31 March to demand proper medical treatment for chronic back pain and numbness in his limbs.
Navalny was detained when he returned to Russia in January after months recovering in Germany from a near-fatal poisoning he blames on the Kremlin - an accusation it rejects.
He was sentenced to two-and-a-half years on old fraud charges his supporters say were politically motivated and has been serving time in a penal colony about 100km east of Moscow.