Russia has launched more than 50 missiles targeting critical infrastructure across Ukraine, including the capital Kyiv, causing power and water outages, Ukraine says.
At least two blasts were reported in Kyiv. One resident told the BBC his district was without electricity.
In the north-eastern city of Kharkiv, energy facilities were hit.
The strikes come after Russia blamed Ukraine for a drone attack on its Black Sea Fleet in the annexed Crimea.
On Monday morning, missile strikes were also reported in the central Vinnytsia region, as well as Dnipropetrovsk and Zaporizhzhia in the south-east, and Lviv in western Ukraine.
A facility at the Dnipro hydroelectric power plant in the Zaporizhzhia region was also reportedly hit.
In Kyiv, a facility which powers 350,000 apartments was damaged, with engineers urgently deployed to restore the supply.
It was not immediately known if there were any casualties.
Residents in the regions under attack were urged to remain in shelters, amid fears more strikes could follow.
Ukraine's Air Force spokesman Yuriy Ihnat told Ukrainian TV that Russia had used its strategic bombers to carry out its "massive" strikes.
Ukraine's military later said that 44 out of more than 50 cruise missiles launched on Monday morning had been shot down.
Ukrainian Foreign Minister Dmytro Kuleba said that "instead of fighting on the battlefield, Russia fights civilians".
Russia has carried out several waves of deadly missile and drone attacks in recent weeks, reportedly destroying almost a third of Ukraine's power stations and other energy-generating facilities ahead of the cold winter period.
Ukraine and its Western allies have repeatedly said that targeting civilian infrastructure amounted to war crimes.
Russia has so far made no public comments on the reported latest strikes.
On Saturday, one Russian warship was damaged in the port city of Sevastopol in a drone attack, the Russian defence ministry said. It also accused British specialists of having trained the Ukrainian soldiers who then carried out the strikes in Crimea - Ukraine's southern peninsula, annexed by Russia in 2014.
Moscow provided no evidence to back its claims.
Ukraine has not commented on the issue, while the UK defence ministry said Russia was "peddling false claims on an epic scale".