The United States and about 50 other countries back Guaido's contention that he is the interim president of Venezuela, and that Maduro's re-election past year was illegitimate because major opposition leaders were barred from running.
Some parts of Caracas and the rest of the country have had their electricity supply restored and Venezuelans are now bracing themselves for a weekend of pro and anti-government protests.
Much of Venezuela plunged into darkness Thursday evening, creating chaos as people struggled to navigate their way home amid what appeared to be one of the biggest blackouts yet in a country where power failures have become common.
However, it was short-lived as the lights went off again, extending the blackout beyond 24 hours.
The nation is experiencing hyperinflation projected to reach a mind-boggling 10 million percent this year, is grappling with food and medical shortages, and has lost about 10 percent of its population to migration in the past few years - including many with valuable energy expertise.
U.S. President Donald Trump's special representative for Venezuela pledged on Thursday that Washington would "expand the net" of sanctions on the South American nation, including more on banks supporting President Nicolas Maduro's government.
Venezuelan opposition protesters confront police blocking their path, in Caracas, Venezuela, Saturday, March 9, 2019.
Speaking on Venezuelan national television, VTV, Electricity Minister Luis Motta Dominguez called the blackout an "electricity war" and act of sabotage.
Why do blackouts happen in Venezuela?Several columns of security forces moved on motorcycles.
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Marielsi Aray, a patient at the University Hospital in Caracas, died after her respirator stopped working.
"The doctors tried to help her by pumping manually, they did everything they could, but with no electricity, what were they to do?" said her uncle Jose Lugo. While some hospitals were able to rely on back-up power sources, others were dark.
The director of CODEVIDA, a coalition of Venezuelan health advocacy groups, reported that thousands of dialysis patients were going without treatment as a result of the blackout.
The putrid odor of rotting flesh hung around the entrance to Caracas' main Bello Monte morgue on Friday where the refrigeration system had stopped working.
Following Mr Maduro's decision to close the borders to keep out humanitarian aid for his people, the country was completely isolated on Friday.
President Nicolas Maduro earlier also raised suspicions of Washington standing behind the power outage as he accused the USA of waging the "electricity war". However, the US Department of State denied any involvement.
Venezuela's normally hyper-active social media was muted during the blackout.
Guaido says Maduro's rule is illegitimate, arguing that his re-election win a year ago was fraudulent, and he wants Maduro to resign from the Miraflores Palace and make way for new elections.
Officials said the hydroelectric station at the Guri Dam, one of the world's largest, had been sabotaged, but offered no evidence. It gave no details.