Ukraine's second city of Kharkiv was for hours left without power after the latest wave of Russian strikes targeted energy stations across the country.
Local officials said as many as nine power facilities were hit as Russian forces fired 76 missiles and carried out drone attacks on Friday.
Kharkiv's mayor said the city suffered "colossal" damage. By the evening, 55% of residents had electricity back.
One resident, Anastaisa, told the BBC the strikes began on Friday morning.
"In a matter of minutes, the lights started blinking," the mother of a two-month-old child said. "Just 10 seconds later, we were out of power, everything just went still and that's it.
"At the moment, there is also no water because the pumping stations cannot work when there's no power in the city, so technically, what we have now is a city without power and water supply."
In the evening, Oleg Synegubov, head of Kharkiv's regional administration, said electricity supplies had been restored to 55% of city residents and 85% of those living in the north-eastern region. He added that energy sector employees were working to restore full power by midnight.
However, Ukraine's power grid operator, Ukrenergo, warned that the scale of the damage could mean it will take longer to restore supplies, reports BBC.
The body said in a statement that the energy system had lost more than half of its capacity because of the latest strikes, adding that priority would be given to "critical infrastructure - hospitals, water supply facilities, heat supply facilities, sewage treatment plants".
Defence Ministry adviser Yuriy Sak told the BBC that emergency services were working to restore electricity supplies, but the situation was "still difficult". Russia's frequent attacks mean that repairing the damage is getting harder.
Elsewhere, three people were killed and 13 more injured when a residential building was hit in Kryvyi Rih and a third died in Kherson. In the capital, Kyiv, the city's metro was left at a standstill.
The alarm was raised across Ukraine on Friday and Commander-in-Chief General Valeriy Zaluzhny said air defences had intercepted 60 of the 76 missiles fired, most of them cruise missiles.
Kyiv city officials said about 40 missiles had been fired at the capital alone - one of the biggest barrages since Russia's 24 February invasion. Thirty-seven were brought down by air defences, the officials added.
"It's very stressful, but now I'm used to this," said 42-year-old Oksana, who lives in the capital. "I don't want our children to live through this, to be in basements, shelters, I don't want this for them."
Ukraine has accused Russia of "weaponising winter" by striking essential facilities, as temperatures in much of the country are below freezing.