Snowstorms and strong winds have left more than 2,000 towns and villages without power across many regions of Ukraine, local officials say.
They say 48 people, including children, have been evacuated from trapped vehicles in the worst-affected Odesa region in the south-west.
At least six people have suffered from hypothermia. Traffic is currently blocked on 14 motorways.
The storm is also battering Moldova, Russia, Georgia and Bulgaria.
In a statement on Monday morning, Ukraine's emergencies service said the bad weather left 2,019 settlements without power in 16 regions of the country.
It said 840 vehicles had to be towed away amid snow drifts reaching more than two metres (6.6ft) in some places. At least 1,370 cargo lorries are currently stuck.
The southern Mykolaiv region is also badly hit.
More than 1,500 rescuers are now deployed across the country in a massive clean-up operation. They are being helped by police, border guards and National Guard members.
In the capital Kyiv, the country's largest flag - measuring 16x24m - had to be taken down from a 90m pole after it was damaged by fierce winds. City officials said the flag would be replaced and raised again.
In Ukraine's southern Crimea peninsula - illegally annexed by Russia in 2014 - Moscow-installed officials reported flooding in coastal areas. Fallen trees and other debris were seen on the streets of several towns.
About 800 exotic fish died when a historic museum-aquarium was destroyed by sea flooding in the port of Sevastopol, the museum director was quoted as saying by a local media outlet.
A state of emergency is now in place in several municipalities in Crimea.
Anton Herashchenko, an adviser to Ukraine's interior ministry, said the storm "washed away trenches in occupied Crimea that Russian army dug out on the beaches".
Russia's military has not commented on the claim.
In Russia's Black Sea city of Sochi, footage has emerged purportedly showing a three-storey building collapsing. No injuries were reported.
Near the town of Anapa, also on the Russian Black Sea coast, a cargo vessel with 21 crew ran aground.
In the capital Moscow, the authorities had to deploy specialist machinery to clear the streets after a heavy snowfall.
The extreme weather comes amid reports that Russia is again preparing massive rocket and drone strikes on Ukraine's power grid and other critical infrastructure.
Last autumn and winter, Moscow carried out waves of such deadly attacks, leaving millions of people across Ukraine without electricity and heating.
Last Saturday, Russia launched its biggest drone strike on Kyiv since its full-scale invasion in February 2022, Ukrainian officials said. They said that 74 out of 75 of Shahed kamikaze drones were shot down around the capital.