Crimea was left without electricity supplies from Ukraine on Sunday after pylons carrying power lines to the Russia-annexed peninsula were blown up overnight.
It was not immediately clear who had damaged the pylons, but a Russian senator described the move as an "act of terrorism" and implied that Ukrainian nationalists were to blame.
Crimea receives the bulk of its electricity from the Ukrainian mainland and its seizure by Russia last year prompted fury in Kiev and the West, which then imposed economic sanctions on Russian companies and individuals.
Russia's Energy Ministry said emergency electricity supplies had been turned on for critical needs in Crimea and that mobile gas turbine generators were being used, adding that around 1.6 million people out of a population of roughly 2 million remained without power as of 1000 GMT.
Ukraine's Energy Minister Volodymyr Demchyshyn said in a statement that four power lines had been damaged and that two districts of Ukraine's Kherson region were also left without power.
Ilya Kiva, a senior officer in the Ukrainian police who was at the scene, said on his Facebook page that the pylons had been "blown up", as did the Kherson region administration.
The head of Crimea, Sergei Aksyonov, declared Monday a non-working day because of the emergency situation in the republic.
The attack, if by Ukrainian nationalists opposed to Russia's annexation of Crimea, is likely to further increase tensions between Russia and Ukraine.
Unidentified people attacked power lines leading to Crimea on Friday, after which a group called the Civil Blockade of Crimea prevented Ukrainian energy officials from conducting repairs.
The group, in which Crimean Tatar activists play a prominent role, denied it was responsible for either the attacks on Friday or Saturday night when contacted by Reuters on Sunday.
In September, Tatar activists opposed to Russia's annexation of their indigenous homeland set up road blocks on roads leading from Ukraine to Crimea as part of an economic blockade aimed at dramatizing the plight of Tatars in Crimea.
On Saturday, the pylons damaged on Friday were the scene of violent clashes between paramilitary police and Tatars as well as members of the nationalist group Right Sector, Russian media reported.
A Ukrainian police representative from Kherson region told TASS news agency on Sunday that police had blocked off the area surrounding the damaged pylons and that preparations were under way for them to be repaired.
Ukraine's Energy Ministry said one of the four damaged power lines could be repaired within 24 hours if workers were allowed safe and unfettered access to the site.
State-run energy firm Ukrenergo said it hoped to finish repairs on all the damaged lines within four days.