Vladimir Putin warns the West not to cross a “red line” with Russia, saying such a move would trigger an “asymmetrical, rapid and harsh” response.
The Russian president’s warning came in his annual state of the nation address, amid heightened tension with the West over Ukraine and jailed Putin critic Alexei Navalny.
Meanwhile, hundreds of pro-Navalny protests agitated on Wednesday in eastern cities including Vladivostok, Irkutsk and Krasnoyarsk. The authorities have declared them illegal. Police detained nearly 100 Navalny supporters rallying in several cities.
The anti-corruption campaigner is being treated at a prison hospital in Vladimir, about 180km east of Moscow. He is on hunger strike and his allies say his life is in danger.
Two close aides to Navalny, lawyer Lyubov Sobol and spokeswoman Kira Yarmysh were among those detained.
Mr Putin in his address claimed Western powers constantly try to “pick on” Russia, Belarus and Ukraine.
“The use of unjust sanctions is growing into something more dangerous: a coup attempt in Belarus,” he said.
Russia backs Belarusian President Alexander Lukashenko, who faces huge opposition since claiming re-election last year, in a vote widely condemned as rigged.
“We don’t want to burn bridges, but if somebody interprets our good intentions as weakness, our reaction will be asymmetrical, rapid and harsh,” he said. “We’ll decide for ourselves in each case where the red line is.”
Tensions have been building over Ukraine, as reports say Russia has moved more than 100,000 troops close to disputed areas.
A large part of that force is in Crimea, the peninsula which Russia annexed from Ukraine in March 2014. A senior Ukrainian officer, Gen Serhiy Nayev, estimated the total force to be 103,200 last week.
Russia backs the separatists holding a swathe of eastern Ukraine, and its manoeuvres have fuelled fears of a new Russian military intervention.
In his speech Mr Putin said that “the West didn’t think about Belarus or Ukraine, when the Maidan events were going on there”. Mass protests in Kyiv’s Maidan Square led to Ukraine’s pro-Russian President Viktor Yanukovych fleeing to Russia in February 2014.
“The organisers of any provocations against Russia will regret [their actions] in a way they never have before,” Mr Putin warned.
Mr Putin’s spokesman Dmitry Peskov later described the “red lines” as “our external security interests, our internal security interests in preventing any outside interference, whether in our elections or other domestic political processes”.