"We can't take in any more corpses", an employee said on condition of anonymity.
Venezuela's electrical system was once the envy of Latin America but it has fallen into disrepair after years of poor maintenance and mismanagement.
The blackout started late Thursday, intensifying a deepening economic and political crisis.
Venezuelans struggling to put food on the table anxious that the few items in their fridges would spoil. Several deaths have been reported.
The mounting political pressure comes as services slowly returned to normal in Caracas and the states of Miranda and Vargas, home to the country's global airport and main port.
But Guaido has avoided discussion of any foreign troops in Venezuela, and Latin American neighbors have emphatically opposed a USA intervention as a way of addressing the situation.
The Caracas subway, which transports two million people a day, remained suspended early Saturday and shops were closed, but internet and telecommunications services were returning to normal. Others had to walk long distances to get home.
Marielsi Aray, a patient at the University Hospital suffering from an aggravated infection, died at dawn on Friday after her respirator stopped working, her uncle Jose Lugo said. "And of course, there's no water".
When more generators were brought to supply emergency power, they weren't enough.
Emilse Arellano said urgent dialysis for her youngest child had to be canceled Friday, after a night during which staff worked by the light of cellphones. Traffic lights in several neighborhoods were back on, while at one office building the electricity flickered to life and then turned off.
The Socialist Party called for a competing march on Saturday to protest against what it calls imperialism by the United States, which has levied crippling oil sanctions on Nicolas Maduro's government in efforts to cut off its sources of funding.
Venezuela has become mired in a major political crisis since Guaido declared himself the president in January and denounced Maduro as an usurper. "I still don't have power and I lost part of my food", she said, referring to food that spoiled due to the blackout.
As Venezuela is still reeling from a nationwide blackout, Nicolas Maduro's government says the emergency was the result of an external influence.
"It's part of the electric war against the state", the company said on Twitter.
State-owned electricity operator Corpoelec blamed the outage on act of "sabotage" at the Guri Dam, one of the world's largest hydroelectric stations and the cornerstone of Venezuela's electrical grid. Information Minister Jorge Rodriguez described it as a cyberattack on the dam's operating system which signals to machines whether to boost or diminish power based on capacity and demand.
"Once we've finished the tour, the organization in every state, we'll announce the date when all together we'll come to Caracas", said the 35-year-old leader of the legislature, who is recognised as interim president by more than 50 countries, including Ireland.
With normally hyper-active social media eerily silent, residents threw open their windows and banged pots and pans in the darkness, shouting expletives at Maduro in a sign of mounting frustration.
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