President Volodymyr Zelenskiy said Russia had launched nearly 300 drones, 18 ballistic missiles, and seven cruise missiles during overnight attacks on eight Ukrainian regions.
The main targets were energy-generation facilities and substations, Zelenskiy said in a post on the Telegram app.
"Every such strike against life is a reminder that support for Ukraine cannot be stopped. Missiles for air defence systems are needed every day, and especially during winter." Ukrainian air defence units shot down 247 drones and seven missiles, the air force said.
It also said on the Telegram app that missiles and drones caused damage at 24 sites. Emergency power cuts were introduced in the capital Kyiv and also in the Chernihiv, Odesa, Kharkiv, Dnipropetrovsk, Zaporizhzhia and Donetsk regions, the Energy Ministry said.
DTEK, Ukraine's largest private energy company, said the attack heavily damaged equipment at one of its thermal plants.
Zelenskiy, in his nightly video address, said emergency crews, hampered by "extraordinary challenges" and temperatures plunging to -16C in Kyiv, remained engaged round-the-clock to restore heating and power.
"It is important that so many of our people are working professionally, restoring supplies and striving to sustain normal life," he said.
Kyiv Mayor Vitali Klitschko posted a video on Telegram of crews repairing pipes. He said that while heating was now on in most apartment buildings, it still had to be restored in nearly 500 dwellings.
Russia has relentlessly targeted Ukraine's energy system throughout the conflict, claiming attacks on civil infrastructure are justified to impair Ukraine's war effort. Kyiv says Moscow's aim is to inflict suffering to break the national will.
Ukrainian officials say the war's fourth winter is shaping up to be the coldest and darkest yet, with Russian strikes intensified, years of cumulative damage taking a toll and the weather dangerously cold even for Ukraine's climate.
In the city of Kharkiv, 30km from the border with Russia and a frequent target, four people were killed during the attacks when a postal terminal was hit by several missiles and drones, said regional governor Oleh Syniehubov.
"We faced a mass Shahed and missile attack," said Syniehubov, referring to Iranian-designed drones.
"More than 30 people were wounded. Four people were killed on site."
In the southern port city of Odesa, at least five people were injured as a result of the overnight Russian attack, the emergency service said. It reported fires at an unused new building, a fitness centre and a vocational school.
