Power is slowly being restored in Argentina and Uruguay after an unprecedented blackout left some 48 million people without electricity.

An electrical grid that serves both South American countries reportedly “collapsed” at 7:07am Sunday local time, cutting electricity in all of Argentina and Uruguay.

Argentina has more than 44 million people, while Uruguay’s population is about 3.5 million.

“A massive failure in the electrical interconnection system left all of Argentina and Uruguay without power,” electricity supply company Edesur said in a statement.

Company spokesperson Alejandra Martinez said: “This is the first time something like this has happened across the entire country.”

Argentine President Mauricio Macri confirmed Monday morning that electricity had been reinstated to more than 50 per cent of the country.

Uruguay’s energy company, UTE, said that power had returned to 88 per cent of customers.

Argentina’s Energy Secretariat said that it would take several hours for power to be fully restored.

Bars and shops close their doors during the massive energy blackout in Argentina. Photo: Getty

The massive outage left Buenos Aires dark early on Sunday, hobbling public transportation, cutting off water supply and crippling phone and internet communications across the Argentine capital.

Chaos erupted as traffic signals were not working and several Argentine provinces were forced to temporarily delay local elections.

Presidential candidate Alberto Fernandez tweeted: “The President should … give an explanation for what Argentines are suffering.”

Images from social media showed long lines of cars at the few service stations still in operation.

Cars wait in line at a gas station in Buenos Aires. Photo: Getty

“The city is a disaster. There are no traffic lights,” 75-year-old retiree Liliana Comis, from Buenos Aires, told the ABC.

“Stores aren’t open. It spoiled Father’s Day.”

Buenos Aires resident Juan Borges said the power outage could have resulted in fatalities.

“Everything came to a halt. Elevators, water pumps, everything. We were left adrift,” the apartment building resident told the BBC.

“There are some elderly people on the eighth floor, but nothing happened, because the power cut was short. If it had gone on for longer it would have been a whole different story.”

The cause of the outage was still unclear as of Monday, but Argentina’s energy agency said in a statement it had begun an investigation.

A spokesman for Brazil’s power system operator said the outage had not impacted the regional neighbour to the north.

-with AAP