Thousands of Venezuelans are expected to protest once again Saturday as opposition leader Juan Guaido cranks up the pressure on beleaguered President Nicolas Maduro -- in the midst of a sweeping electricity blackout that has crippled the crisis-wracked country. 
Both Guaido and Maduro, who are locked in a bitter power struggle for the right to lead the oil-rich South American nation, have asked their supporters to fill the streets of Caracas and other cities.
"Tomorrow, I call on the Venezuelan people to make a huge statement in the streets against the usurper, corrupt and incapable regime that has plunged our country into darkness," Guaido wrote on Twitter.
"We return to the streets and we won't leave until we reach the goal," said the 35-year-old National Assembly leader, who is recognized by more than 50 countries as Venezuela's interim president.
Guaido is trying to force out Maduro -- whose re-election in May he deems illegitimate -- in order to set up new elections.
Maduro has meanwhile asked his backers to march on Saturday against "imperialism."
The mounting political pressure comes after one of the worst and longest blackouts in recent memory in Venezuela. It began on Thursday afternoon, paralyzing much of the country.
Power was gradually being restored to large areas of Caracas on Friday afternoon, as well as parts of Miranda state and Vargas, which contains the country's international airport and main port.
But the relief was short-lived as the lights went off again, extending the blackout beyond 24 hours.
As night fell, people banging saucepans could be heard around the capital: a form of popular protest often seen in South America.
Maduro has blamed the blackout on US sabotage and shut down offices and schools on Friday.
Large lines formed at the few gas stations open as people fetched fuel for generators. Some took gas from their cars.
The power "came for one moment and went away again," Antonio Belisario, who had been waiting around an hour for petrol, told AFP. 
Venezuela has suffered more than four years of recession that has seen poverty soar as citizens struggle with food and medicine shortages.
So the blackout came at an already stressful time -- and made matters worse.
The outage had left most of the country in disarray, crippling day-to-day functioning of hospitals and other public services, according to local press reports.
Witnesses described scenes of chaos at several hospitals as people tried to move sick relatives in the dark to clinics with better emergency power facilities. 
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?" asked Jose Lugo, her distraught uncle.
Generators at the JM de Rios children's hospital in downtown Caracas failed to kick in when the blackout hit, said Gilbert Altuvez, whose eight-year old son is among the patients.
"The night was terrible. Without light. Total madness," he said.
The putrid odor of rotting flesh hung around the entrance to Caracas' main Bello Monte morgue on Friday where refrigerators had stopped working and worried relatives gathered outside, waiting to be allowed to bury their dead.

The blackout in the capital hit Thursday at 4:50 pm (2050 GMT) -- just before nightfall. Its cause is still unknown.
Thousands of homes in Caracas -- a crime-ridden city of two million people -- were without water. Telephone services and access to the internet were knocked out while the subway system ground to a halt.
The capital's Simon Bolivar international airport was hit, as were others across the country.
Following Maduro's decision to close the borders to keep out humanitarian aid for his people, the country was completely isolated on Friday.
"The electrical war announced and directed by US imperialism against our people will be defeated," Maduro tweeted.
Critics blame the government for failing to invest in maintaining the electrical grid, although the government often blames external factors when the lights go out.
The state power company Corpoelec said there had been sabotage at the Guri hydroelectric plant in Bolivar state, one of the largest in Latin America. It gave no details.
US Secretary of State Mike Pompeo said Maduro was wrong to blame the US or any other country for Venezuela's woes.
"Power shortages and starvation are the result of the Maduro regime's incompetence," he tweeted.
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