President Vladimir Putin warned foreign rivals on Wednesday against "crossing the red line" with Russia, as he gave a key speech amid deep tensions with the West over Ukraine and jailed Kremlin critic Alexei Navalny.
Several thousand Navalny supporters took to the streets in nationwide demonstrations against his detention, but the protests were not on the scale seen earlier this year, when tens of thousands rallied and thousands were arrested.
Addressing lawmakers and senior officials in his annual state of the nation address, Putin said Russia would respond swiftly and harshly to moves against its interests and condemned what he said had been a coup attempt in ally Belarus.
"The organizers of any provocations threatening the fundamental interests of our security will regret their deeds, more than they have regretted anything in a long time," Putin said.
Putin unsurprisingly made no mention of Navalny in his speech -- he has always refused to use the name of his most prominent opponent.
- 'New kind of sport' -
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.
Putin said it had become "a new kind of sport" in some foreign capitals to blame Russia "for anything".
He said Russia wants good relations with everyone, but warned of an "asymmetrical, swift and harsh" response if that was seen as a 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," Putin said.
Putin also backed claims by Belarusian President Alexander Lukashenko that his security services had thwarted an alleged US plot to assassinate him, suggesting there had been a "coup attempt" and accusing the West of pretending "that nothing is happening".
Putin is due to meet Lukashenko -- who has faced down historic protests since a disputed re-election last summer -- in Moscow on Thursday, amid speculation of a major announcement on Russia's policy towards its ex-Soviet neighbor and ally.
Much of Putin's address was devoted to new social spending, as the longtime leader looks to shore up support for his deeply unpopular United Russia party ahead of parliamentary elections in September.
"The main thing is to ensure the growth of citizens' real incomes," he said, announcing new financial support schemes for families.
- More than 400 arrested -
Navalny's supporters were hoping to steal Putin's thunder with a series of mass protests starting from 7 pm in cities across the country.
They are demanding that Navalny, who launched a hunger strike three weeks ago, be freed or at least given proper medical treatment after his doctors said his health was failing and he could die "at any minute".
Demonstrations took place in cities including Vladivostok in the Far East, and Novosibirsk, Irkutsk and Tomsk in Siberia, before starting in Moscow and Saint Petersburg.
Security was tight in Moscow, with areas around the Kremlin and parts of the city centre sealed off by riot police.
Shortly after the demonstrations began, AFP journalists said several thousand people had gathered in Moscow and Saint Petersburg, waving Russian flags and chanting "Freedom!"
"I want, at best, for Navalny to be freed, but if not that he has access to medical care," said 54-year-old protester Olga Yelagina.
"If a lot of people gather... maybe we can change something in Russia."
Police had issued a warning against taking part in "illegal gatherings" and detained at least 400 people across the country, according to monitoring group OVD-Info.
Among those detained in Moscow were Navalny ally Lyubov Sobol and his spokeswoman Kira Yarmysh.
- Calls for medical evacuation -
Navalny was arrested when he returned to Russia in January after months recovering in Germany from a near-fatal nerve agent 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 100 kilometres (60 miles) east of Moscow.
His health has been failing since he launched his hunger strike to demand proper medical care for a range of ailments.
The European Union and the United States have imposed sanctions on Russia over Navalny's poisoning, and on Monday threatened Moscow with further penalties in the event of his death.
A group of UN rights experts expressed alarm Wednesday over his deteriorating health.
"We believe Mr Navalny's life is in serious danger," they warned, calling on Russian authorities to allow Navalny "to be evacuated for urgent medical treatment abroad."