The Ukrainian authorities have warned people in the capital Kyiv to expect longer power cuts, lasting more than four hours, because of Russian attacks on energy infrastructure.
Rolling blackouts are hitting not only Kyiv but also central regions of Ukraine, including the city of Dnipro.
President Volodymyr Zelensky said about four million people were affected but “shelling will not break us”.
This month Russia launched dozens of missiles and Iranian-made drones.
Ukraine’s energy infrastructure is being pounded by the air attacks – Mr Zelensky says about a third of the country’s electric power stations have been destroyed.
The Kyiv region has lost 30% of its power capacity, the private energy company DTEK says, meaning “unprecedented” power cuts will be necessary.
“Unfortunately the scale of restrictions is significant, much larger than it was before,” said DTEK director Dmytro Sakharuk.
The power cuts have meant curbs on the use of street lights and electric-powered public transport, besides the discomfort in people’s homes.
The EU and other international allies of Kyiv have condemned the deliberate targeting of civilian infrastructure – attacks that Ukraine sees as war crimes.
Ukraine’s second city Kharkiv, heavily damaged by Russian shelling, also faces long power cuts, along with the central cities of Zhytomyr, Poltava and Chernihiv.
Russia stepped up its missile attacks on Ukraine’s power stations and other civilian infrastructure in retaliation for the 9 October bombing of the Crimean Bridge – a key link to Russian-annexed Crimea.
President Vladimir Putin called that blast a Ukrainian “act of terrorism”. The bridge is a symbol of his campaign to incorporate large swathes of Ukraine into Russia.
A power station employee called Pavlo, quoted by AFP news agency, said “we are confronted by such damage for the first time”. The unnamed plant had twice been targeted by missiles and then by an Iranian-made “kamikaze” drone.
He said repairs had been under way for more than two weeks, but “there are difficulties in that the equipment that has been damaged is unique – it’s hard to find the same parts”.
In other developments: