Russia launched one of its largest attacks against Ukraine since the start of its war on Friday, pounding the country’s power grid with scores of missiles.
Two people were killed in the barrage, which damaged nine energy facilities and forced Kyiv to introduce emergency blackouts across the country as winter bites.
Many people headed for shelters during the morning rush hour to take cover.
The attack was the largest against vital infrastructure since October, and one of the biggest missile barrages since Russia invaded in February, according to a Kyiv official.
The mayor of Ukraine's second-largest city Kharkiv, in the north-east, reported "colossal" damage, threatening to leave many people without heating in freezing winter temperatures.
The governor of the central region of Dnipropetrovsk reported "serious damage".
Economy Minister Yulia Svyrydenko wrote on Facebook: "The goal of the Russian Federation is for Ukrainians to be constantly under pressure, to go down into bomb shelters almost every day, to feel discomfort due to power outages or water interruptions.
"But Ukraine's position is unchanged: let it be without light, but #withoutyou. We will endure. We will win. We will rebuild."
Air defences took out 60 out of 76 incoming missiles fired at critical infrastructure, Ukraine's top general said.
Moscow had tried to distract the air defences by flying warplanes near Ukraine, Air Force spokesperson Yuriy Ihnat said.
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"They want to destroy us, and make us slaves. But we will not surrender. We will endure," said Lidiya Vasilieva, 53, as she headed for shelter at a Kyiv railway station.
"I want the war over, and soon. But I am ready to wait as long as needed."
Two people were killed in the central city of Kryvyi Rih, and a third died in a fire in the southern Kherson region after an apartment block was hit by shelling before a missile strike, authorities said.
The attack in Kryvyi Rih, President Volodymyr Zelenskyy’s home city, also wounded at least eight people, including three small children, and a rescue effort was still under way, a senior official said.
Grid operator Ukrenergo warned that repair times would be longer compared with previously.
"What we already see is damage to about nine generating facilities. Now we are still verifying the damage," Energy Minister German Galushchenko told national television.
The central city of Poltava and parts of Kyiv were among areas where power was knocked out. Critical infrastructure was hit in the Kharkiv region, the Black Sea region of Odesa and in Vinnytsia in west central Ukraine.
Three loud blasts were heard in Kyiv and smoke was seen overhead, witnesses said. Some railway lines were left without power.
"There is colossal damage to infrastructure, primarily the energy system," Kharkiv Mayor Ihor Terekhov said.
"I ask you to be patient with what is happening now. I know that in your houses there is no light, no heating, no water supply."
Kyrylo Tymoshenko, deputy head of Mr Zelenskyy's office, said emergency power shutdowns had been introduced nationwide to enable repairs.
The initial air raid alert in Kyiv, announced during morning rush hour, was lifted four hours.
The cavernous Kyiv metro system, which has one of the deepest stations in the world, stopped functioning and was used as a bomb shelter.
Russia, which invaded Ukraine in February, Moscow says attacks on basic infrastructure are militarily legitimate. Ukraine says attacks intended to cause civilian misery are a war crime.