"There has been a total disconnection of the Electric System," the Energy Ministry and National Electric Union said.
The grid operator said the grid collapsed at 9.14am on the Caribbean island.
The ministry said causes are being investigated and the restoration process has already begun.
Even before Wednesday's grid collapse, leaving 10 million residents without power, many across the island had already been experiencing daily blackouts of 16 hours or more.
The grid failure follows a string of blackouts since late last year that plunged Cuba's frail and antiquated power generation system into near-total disarray.
The country has also been facing fuel, food and other shortages amid its worst economic crisis in decades.
"This is crazy for everything," Raúl Ernesto Gutierrez said in Havana.
Gutierrez, who said he was visiting the capital, said that back home in the countryside, "we will have to cook with charcoal, with firewood. It's stressful and also frustrating".
Havana state worker Danai Hernandez said she was on the way home from work which had just shut down.
"I'm going home to organise everything in the household and ... now we have to wait. We don't have any other choice," she said, visibly upset.
Cuba's oil-fired power plants, already obsolete and struggling to keep the lights on, reached a full crisis last year as oil imports from Venezuela, Russia and Mexico dwindled.
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