President Vladimir Putin warned the West on Wednesday (April 21) not to cross Russia's "red lines," saying Moscow would respond swiftly and harshly to provocations.
Relations are under acute strain over Ukraine and the health of jailed opposition leader Alexei Navalny, whose supporters were gathering in protest across the country as Putin spoke.
Putin makes a point of never naming Navalny, his fiercest critic, who is three weeks into a hunger strike in jail and gravely ill.
"We don't want to burn bridges," Putin said in his annual state-of-the nation address. However:
"Those who organize any provocations that threaten our fundamental security interests will regret about their actions like they have not regretted about anything for a long time."
Monitors said police seized scores of protesters in the first hours of the rallies.
Before them, two of Navalny's closest allies were arrested in Moscow, their lawyers said. They were Lyubov Sobol, one of the faces of Navalny's popular YouTube channel, and Kira Yarmysh, his spokeswoman.
The EU called the arrests "deplorable."
Navalny survived a nerve-agent poisoning last year that Russia denies carrying out. He was sentenced in February to two and a half years for parole violations related to an embezzlement case.
Navalny says the charges are politically motivated.