Fidel Castro once called Cuba’s power plants “prehistoric.” But the country still relies on them, contributing to the longest blackout since the collapse of its former patron, the Soviet Union.

The Cuban government has announced emergency measures to slash electricity demand, including suspending classes, shutting down some state-owned workplaces and canceling nonessential services. Officials said that 1.64 gigawatts went offline during peak hours, about half the total demand at the time.

Millions of Cubans remain without electricity because of a prolonged outage at the main power plants on the island.