Tensions have soared between Russia and Ukraine since Crimea was annexed in March 2014 after Russian troops wearing uniforms with no identifying markings seized key infrastructure, and pro-Russian leaders held a snap referendum to secede from Ukraine.
"Crimea has been fully shut off," said Viktor Plakida, head of Crimea's state energy company, the Russian state news agency TASS reported. Russian officials said they were using mobile generators to return power to crucial infrastructure, including hospitals.
Pro-Ukrainian activists are staging a "blockade" of Crimea near the site of the transmission towers, and have urged the Ukrainian government to halt deliveries of food to the peninsula. Ukrainian news agencies said the protesters denied that they had attacked the towers.