More than two third of Kyiv was still without power on Thursday morning one day after Russian strikes battered Ukrainian cities.
Electricity was partially restored in the western city of Lviv and Ukraine's second-largest city, Kharkiv.
The outages are the biggest the capital has experienced since the beginning of Russia's invasion began on February 24.
According to Kyiv Mayor Vitali Klitschko, electricians and repair workers were doing everything to get the power back on "as fast as possible", but the recovery would depend largely on the overall energy "balance" of the nationwide grid.
Officials say ten people were killed in the missile attacks on Wednesday, which knocked out power and water service in several regions.
Ukraine was also forced to disconnect three nuclear power plants -- Rivne, South Ukraine, and Khmelnytskyi -- from the national electricity grid as a precaution, state energy company Ukrenergo wrote on Telegram.
"After yesterday's massive bombing, energy workers were able to reconnect three nuclear power plants to the electricity grid on Thursday morning," Ukraine's energy ministry later said.
Supply was also halted to Europe's largest nuclear plant in Zaporizhzhia, which is still under Russian control.
"Several million people without energy supply, without heating and without water, this is obviously a crime against humanity," Ukrainian President Volodymyr Zelensky told the UN Security Council. "Hospitals, schools, transport, residential districts all suffered."
In his nightly video address, Zelenskyy also pledged that Ukraine would rebuild infrastructure damaged and stated that Ukrainians were "an unbreakable people".
Kyrylo Tymoshenko, deputy chief of President Volodymyr Zelenskiy's staff, said power supplies had been restored in the Kirovohrad and Vinnytsia regions.
In the south, Mykolaiv region governor Vitali Kim appealed to Ukrainians to be as frugal as possible in their use of power.
"Consumption has been growing this morning (which is logical), there isn't enough capacity in the system to switch it on for more consumers," he wrote on Telegram.
"The energy system is united like we all are! If you've turned off a few unneeded lights, that's really important."
According to the Ukrainian air force, Russia sent 10 suicide drones and fired 67 cruise missiles into Ukraine on Wednesday.
Russia has repeatedly targeted Ukraine's energy infrastructure in recent weeks ahead of the winter and following recent battlefield setbacks.
Temperatures in the country plunged below zero overnight, with snow and ice already reported in Kyiv.
Moscow says the aim of its missile strikes is to weaken Ukraine's ability to fight and push it to negotiate.
Russia's latest strikes on Ukraine also caused power outages in neighbouring Moldova, but power had been restored to most areas on Wednesday evening.
Premier Energy, a Moldovan company that serves the centre and the south, said 90% of the electricity supply was back online.