President Volodymyr Zelenskiy said Russia had launched 289 drones, 17 ballistic missiles and 59 cruise missiles overnight, hitting power plants and substations from the Black Sea port of Odesa in the south to Rivne in the west.
Ukrainian Energy Minister Denys Shmyhal said Russian forces had also deployed three drones in the direction of a substation vital to the operations of the Khmelnytskyi nuclear power plant, "creating a threat not only to Ukraine but also to nuclear safety throughout Europe".
Meanwhile, Russian officials said Ukraine had set fire to a major oil refinery in the city of Yaroslavl overnight.
Ukrainian officials said the onslaught was one of the largest of the war, causing power cuts across the capital Kyiv and at least six other regions, and disrupting water supplies and transport in the capital.
The latest attacks, less than two days after the end of a brief humanitarian truce in the Russian-controlled town of Oleshky, do not bode well for the talks the US has scheduled for Abu Dhabi on Tuesday and Wednesday.
At a press conference in Berlin with German Chancellor Friedrich Merz, Zelenskiy said the attack showed Moscow was intent on killing people instead of building on the humanitarian ceasefire in and around Oleshky.
"It is very telling that all of this is happening on the day of the US congressional elections," Zelenskiy said, appealing to allies for additional air defences.
"The Russians believe that America will not notice this signal from them. But such terror must be noticed and stopped."
Ukraine's largest private generating company, DTEK, said three of its five thermal power plants had been knocked offline.
Kyiv Mayor Vitali Klitschko described the situation with power in the city as "extremely difficult".
Russia, he said, was "continuing to destroy Ukraine's unified energy system", damaging generating capacity and transmission lines.
With memories still fresh of last winter's bitter cold, when Russian strikes left millions without power and heating for days at a time, many Ukrainians fear the months ahead.
Zelenskiy also said the death toll from Russia twice bombing a school being used for aid distribution near the front line in Ukraine's Zaporizhzhia region on Monday had risen to at least 33 people, with about 50 people believed to still be buried under the rubble.
Germany has become Ukraine's largest supporter in its war against Russia's invasion since Donald Trump returned to the US presidency last year and withdrew US financial support.
The European Commission said it would host a meeting on Ukraine on Wednesday with the US, NATO and European allies, as well as Ukraine itself.
Zelenskiy said they would discuss a "prosperity plan" and security guarantees for Ukraine.
The transatlantic alliance said it would host further consultations on Thursday.
