Russian attacks left Ukraine's southeastern regions of Dnipropetrovsk and Zaporizhzhia almost entirely without power, the country's energy ministry said on Wednesday night.
Critical infrastructure was operating on reserve power, it said in a statement on Telegram, while officials said water supplies and the internet were also disrupted.
Russia has recently intensified attacks on Ukraine's energy infrastructure, aimed at paralysing power supplies during a harsh winter.
Ukraine's energy system is under enemy attacks every day, and energy workers are operating in extremely difficult conditions to provide people with light and heat, Ukraine's Prime Minister Yulia Svyrydenko wrote on Telegram.
Deteriorating weather conditions put additional strain on critical infrastructure, she said.
The disruptions to power and heating have come as temperatures plunge.
Emergency restoration work in the regions affected by the attack will begin immediately as soon as the security situation allows, state energy company Ukrenergo said on Telegram.
Dnipro city Mayor Borys Filatov said on Telegram all city hospitals had been completely switched to generators.
School holidays had been extended until 9 January because of the power cuts, he added.
The head of the Zaporizhzhia regional administration, Ivan Fedorov, said everything possible was being done to restore power.
DTEK, Ukraine's biggest private energy provider, is living in permanent crisis mode because of Russian attacks on the grid, its chief executive told the BBC last month.
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