Three-quarters of Crimea's population remain without power after four electricity pylons were blown up.
Gas-powered generators have been providing power to major cities. A state of emergency has been declared.
The pylons brought electricity from Ukraine. Engineers were reportedly denied access to the site by Ukrainian activists.
Crimea was annexed by Russia last year, but the Ukrainian authorities have continued to supply power to the area.
Images on social media show Ukrainian flags on some damaged pylons - and Crimean Tatar flags on others.
Crimean authorities said they had managed to partially reconnect the cities of Simferopol, Yalta and Saky using generators.
But more than 1.6 million people remain without power, and water supplies to high buildings have stopped and cable and mobile internet is down. Some 150 schools were also without power.
"I had no electricity all night. These useless officials can't run the city and they still haven't built a local power station," a resident of Sevastopol told AFP news agency.
"It's not the first time Ukraine has cut off electricity to Crimea, we are already used to power cuts and stock up on batteries," another one said.
Mikhail Sheremet, Crimea's deputy prime minister, said the peninsula's hospitals had backup power sources and would not be affected.
Two of the four main power lines were cut in an earlier attack on Friday, reports said.
Ukrainian authorities said they encountered activists blockading the site when they tried to repair the damaged pylons.
"The nature of the damage shows that it took place as a result of shelling or the use of explosive devices," Ukraine's state energy company Ukrenergo said in a statement.
Ukrenergo said it hoped to finish all repairs within four days.
Crimean Tatars, an ethnic group native to the peninsula who oppose Russian rule, held a protest at the site of the broken power lines in Kherson region, Russia's RIA news agency reported.