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.