Russian President Vladimir Putin declared martial law in the four regions of Ukraine that Moscow illegally annexed and given additional emergency powers to the heads of all regions of Russia.
Mr Putin did not immediately spell out the steps that would be taken under martial law, but said his order was effective starting on Thursday.
His decree gives law enforcement agencies three days to submit specific proposals.
The upper house of Russia’s parliament is set to quickly seal Mr Putin’s decision to impose martial law in the four regions.
Draft legislation indicates it may involve restrictions on travel and public gatherings, tighter censorship and broader authority for law enforcement agencies.
Mr Putin also did not provide details of the extra powers to be given to the heads of Russian regions under his decree.
“In the current situation, I consider it necessary to give additional powers to heads of all Russian regions,” he said.
The Russian leader also ordered the establishment of a Coordination Committee to increase interaction between various government agencies in dealing with the fighting in Ukraine that he continued to call a “special military operation”.
In televised remarks at the start of a security council meeting, the Russian leader said: “Those who are on the frontlines or undergoing training at firing ranges and training centres should feel our support and know that they have our big, great country and unified people behind their back.”
Mr Putin’s move came after Russian forces launched mass evacuations of civilians from one of the first major cities they seized in the invasion of Ukraine – a tacit acknowledgement that yet another stinging battlefield defeat may be unfolding for the Russian leader.
Ukraine’s stunning counter-offensive appears to be bearing down on Kherson, a southern city of more than 250,000 people, with industries and a major port on the Dnieper River.
The battle for the city is a pivotal moment for both Ukraine and Russia heading into winter, when cold and difficult conditions could largely freeze front lines until the spring thaw.
What had previously been a trickle of evacuations from the city in recent days was becoming a flood.
Residents could be seen on Russian state television crowding on the Dnieper’s banks, many with small children, to cross by boats to the east – and, from there, deeper into Russian-controlled territory.
Text messages warned residents to expect shelling and said buses were being prepared for them to leave, Russian state media reported. Leaflets told evacuees that they could take the weight equivalent of two large suitcases, medicines and food for a few days.
Moscow-backed authorities have said evacuations from occupied territories are voluntary. But in many cases, the only routes out are to Russia.
Vladimir Saldo, the region’s Moscow-installed head, said: “It’s better to evacuate people in case the fighting starts to involve artillery shelling and bombing of the city, and that’s what we are doing now.”
Andriy Yermak, head of the Ukrainian presidential office, called the evacuation “a propaganda show” and said Russia’s claims that Kyiv’s forces might shell Kherson were “a rather primitive tactic, given that the Armed Forces do not fire at Ukrainian cities”.
Ukrainian forces have rolled back Russian positions on the river’s west bank in recent weeks, and the region’s Moscow-installed administrators now appeared to be hoping that the Dnieper’s wide, deep waters will act as a natural barrier against those approaching forces.
In a rare acknowledgement of the pressure that Kyiv’s troops are exerting on the ground, Russia’s new commander for Ukraine on Tuesday described the situation for Russian forces in the Kherson region as “very difficult”.
Russian bloggers have interpreted General Sergei Surovikin’s comments as a warning of a possible pullback of Moscow’s forces.
Kherson is one of four partly or fully-occupied regions that Russia illegally annexed last month, in an effort – widely condemned and rejected by Western nations – to cement its land-grabs.