Russia has fired a barrage of missiles at targets across Ukraine for the eighth time in eight weeks.
Significant disruption to the power grid was reported, mainly in the east. In the south, Odesa was without power. Ukraine says four people were killed.
Ukraine says it shot down 60 of the 70 missiles fired by Russia. Moscow says it hit all 17 of its targets.
In his video address late on Monday, Ukrainian President Volodymyr Zelensky said the strikes had also hit electricity supplies in neighbouring Moldova.
"This once again proves that Russia's ability to carry out such massive terrorist attacks is a threat not only to Ukraine, but also to our entire region."
Previous Russian attacks have hit Ukraine's energy grid, leaving millions without electricity and heat as winter arrives.
Warnings that Russia was planning a fresh wave of attacks have been circulating for several days. They eventually arrived just hours after a series of explosions at two airbases deep inside Russia, which Moscow blamed on Ukrainian drones intercepted by Russian air-defences.
Russia blames Ukraine for attacks on two airfields
For Ukraine's hard-pressed electrical engineers, Monday's missile strikes are likely to mean another round of emergency repairs, often to facilities hit repeatedly in recent weeks.
Before today's strikes, officials in Kyiv were talking about moving from highly disruptive emergency blackouts, which often last for many hours, to more manageable scheduled power cuts which offer civilians some badly needed predictability.
Those plans may now be on hold, but it seems Monday's damage could be less extensive than in previous attacks.
Another striking feature of the latest Russian strikes is the apparent absence of Iranian-made "kamikaze" drones.
Ukrainian officials recently said Russia had used up its supply of the unmanned vehicles. Another Ukrainian military official said cold weather had prevented Russia from deploying them.
Either way, they do not appear to have been used since mid-November.
Russia began its large-scale, co-ordinated attacks on Ukraine's power grid on 10 October. Since then, around half of the country's energy infrastructure has been damaged, leaving millions of Ukrainians disconnected from electricity just as winter temperatures dropped below zero.
Is attacking Ukraine's power grid a war crime?
Some Western leaders have called the strategy a war crime, because of the huge amount of damage caused to civilian infrastructure. Attacks on power grids are not necessarily violations of international law, but they do have to be proportionate to any military advantage gained.
Experts have told the BBC that Russia's tactic of hitting energy infrastructure is most likely designed to demoralise and terrorise the population, rather than gain any concrete military advantage. This would be a violation of international law, as set out by Addition Protocol I to the Geneva Conventions.