Venezuelan opposition leader Juan Guaido on Monday called for a new mass demonstration as the country's devastating blackout that has left millions without power entered its fifth day.

Parliament accepted the 35-year-old's request to declare a "state of alarm" to pave the way for the delivery of international aid, 250 tons of which has been stuck for a month at Venezuela's borders with Colombia and Brazil.

Guaido and the opposition-controlled legislature have no means to enforce it, though, as Maduro controls the military and security services currently preventing aid from entering the country.

Guaido, recognised as Venezuela's interim president by more than 50 countries, called on the military and security services to "refrain from preventing or hindering" Tuesday's protests.

Describing the situation as a "catastrophe," he said the blackout -- the worst in the Latin American country's history -- had claimed "dozens" of lives since it began on Thursday.

Power was restored to some areas of the country over the weekend but -- with residents and businesses fearing that refrigerated food would spoil -- service was patchy and power often lasted just a few hours before dropping out again.

Even the National Assembly's emergency session fell victim to the blackout, as the electricity supply failed an hour into the meeting.

"We cannot turn away from this tragedy our country is going through," said Guaido, who declared himself acting president in January, triggering a power struggle with Maduro in the oil-rich South American country of 30 million.

Businesses and schools remain shuttered on Maduro's orders, as they have been since the blackout began. In any event, the lack of public transport made travel difficult, even in Caracas.

Maduro extended the closure for another 24 hours on Monday due to the blackout, the second time he has done so.

A ball of flame flared across the darkened sky over southeastern Caracas in the early hours of the morning when the Humboldt power station erupted in a massive explosion, increasing the chaos in an area where looting was reported on Sunday.

"I felt like I was in a horror movie," said Carolina Molina, who witnessed the blast from her window.

State-owned electricity company Corpoelec declined to comment on the explosion.

The fire that followed the blast was still smoldering by mid-morning. A metal beam holding one of the huge transformers had melted and a wall had been blackened, AFP reporters said.

The power crisis may spell trouble for Maduro's support from the military, the Eurasia Group consultancy said.

"The government's inability to resolve the crisis and the resulting impact on oil exports, along with challenging social dynamics, may start to shift the military's calculations about the sustainability of Maduro's regime," it said.

Maduro has ordered that hospitals -- where patients were languishing without proper treatment, and where at least 15 people reportedly died due to a lack of kidney dialysis treatment -- should be given top priority.

He has blamed the blackout on US "sabotage" in the form of an electromagnetic attack on the country's main hydroelectric complex in Guri, which supplies 80 per cent of Venezuela's electricity.

Guaido dismissed that explanation as "Hollywoodesque." Critics blame the government for failing to maintain the power grid, as did the Lima Group, a primarily Latin American bloc.

For ordinary Venezuelans, the blackout has piled misery upon an already agonizing day-to-day struggle to survive in a country in economic freefall.

Venezuela has been in recession for more than four years, has struggled with hyperinflation the International Monetary Fund says will hit a staggering 10 million per cent this year, and seen a mass exodus of an estimated 2.7 million migrants since 2015.

"Every day is worse," said Edward Cazano, a 20-year-old who lives with his mother and three brothers in a poor Caracas neighbourhood called Pinto Salinas.

"We have the worst services in the world: no light, no water, sometimes no gas.

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