Cuba’s national power grid shut down on Friday, leaving around 10 million people on the island without electricity.
Cuban President Miguel Diaz-Canel said officials were giving “absolute priority” to fixing the problem.
“There will be no rest until it is restored,” he added.
The government on the island closed schools and “non-essential” industrial sites on Friday morning in a last-ditch effort to preserve supplies.
But shortly before midday, the Antonio Guiteras power plant – one of Cuba’s biggest – went offline, causing a total grid failure.
Officials did not go into details about what caused the shutdown or say how long it might take to get the site back online.
All “non-vital” government services, along with schools and universities, are expected to remain closed until at least Sunday. Cultural and recreational sites, including nightclubs, have also been ordered to shut.
However, not everyone on the island has been left without power.
Some homes and businesses, such as restaurants, have private generators.
The US’s Guantanamo Bay naval base, on the island’s southeastern coast, is also thought to be unaffected because it has its own power supply.
It comes following rolling power cuts across the country in recent weeks.
Cuban Prime Minister Manuel Marrero said on Thursday the issues had been caused by a combination of infrastructure problems, fuel shortages and rising demand, including from household air conditioning units.
“The fuel shortage is the biggest factor,” he said in a televised address.
Residents in Havana interviewed by Sky News’ US partner NBC News expressed dismay at the latest outage.
One man told the broadcaster: “This is incredible. I don’t see a solution to this problem.”
A woman added: “It really worries me that we may not yet be at the bottom of this electricity crisis.”
Brazilian tourist Carlos Roberto Julio also expressed frustration.
He told news agency Reuters: “We went to a restaurant and they had no food because there was no power, now we are also without internet.
“In two days, we have already had several problems.”
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Officials said strong winds from Hurricane Milton last week had hampered efforts by boats to deliver fuel needed to run its power plants.
A US trade embargo, along with sanctions introduced by former president Donald Trump, has also been blamed for reducing supplies of fuel and spare parts needed to repair machinery in Cuba’s power plants.
President Diaz-Canel said in a statement earlier this week: “The complex scenario we are going through has its main cause in the intensification of the economic war and financial and energy persecution by the United States, which makes it difficult to import fuel and other resources necessary for this industry.”
The White House has hit back at the claims. A US national security council spokesperson said: “The United States is not to blame for today’s blackout on the island or the overall energy situation in Cuba.”
Cuba’s largest oil supplier, Venezuela, has added to the problems by reducing shipments to the island this year as it grapples with its own shortages.
Russia and Mexico have also significantly reduced supplies to the Caribbean country.