Power has begun to trickle back into the grid after a massive blackout hit the South American countries of Argentina, Uruguay and Paraguay, leaving tens of millions without power, authorities said.

Key points: The blackout hit at 7:00am on Sunday (local time) across all three countries

The blackout hit at 7:00am on Sunday (local time) across all three countries Power was starting to be restored more than five hours later to some major cities

Power was starting to be restored more than five hours later to some major cities Shops closed and traffic lights stopped working, creating chaos

Argentina's grid "collapsed" about 7:00am local time, leaving the entire country without power, Argentina's Energy Secretariat said in a statement.

The outage also cut electricity to swaths of neighbouring Uruguay and Paraguay.

Energy distributors in Argentina, Paraguay and Uruguay, whose populations total nearly 55 million, said power was being restored to major cities — including Montevideo and Buenos Aires after being without power for at least five hours.

An employee stands in the closed entrance of a subway in Buenos Aires. ( AP: Tomas F Cuesta )

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Argentine President Mauricio Macri called the massive blackout "unprecedented", and promised a thorough investigation.

He said it had been prompted by a failure in the country's coastal grid, but officials still did not know what had caused the problem.

Argentina's energy agency said in a statement it had begun investigating the causes of the outage, but had not provided further details.

Energy company Edesur Argentina said it had returned service to 450,000 of its clients in the greater Buenos Aires area by noon local time, but called the outage "exceptional" and said it would likely take the rest of the day before power was completely restored.

Uruguay power company UTE said on social media that power had returned to parts of Montevideo and the southern coast of Uruguay.

In Paraguay, the capital of Asuncion was unaffected by the outage but local providers said they were restoring power to smaller cities and rural areas.

Blackout cripples cities and towns

Buenos Aires's subways were abandoned after the blackout put a stop to all services. ( AP: Tomas F. Cuesta )

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The massive blackout left Buenos Aires dark early this morning, hobbling public transportation, cutting off water supply and crippling phone and internet communications across the city.

Images from social media showed long lines of cars at the few service stations still in operation in Argentina's capital city and traffic lights dark, creating chaos in places even on a normally quiet Sunday.

"The city is a disaster. There are no traffic lights," 75-year-old retiree Liliana Comis, from Buenos Aires, said.

"Stores aren't open. It spoiled Father's Day."

One Twitter user said "never in my life have I seen such a gigantic power cut", while another posted a picture of dark streets, asking: "What is happening with this blackout?"

The outage also spilled over into local and national politics.

Voters cast ballots by the light of mobile phones in gubernatorial elections in Argentina. One news outlet tweeted that elections had been delayed due to the power outage but would resume once there was sunlight.

Patients dependent on home medical equipment were urged to go to hospitals with generators.

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Alberto Fernandez, a Peronist presidential candidate looking to unseat incumbent Mauricio Macri in this year's hotly contested election, tweeted: "The President should … give an explanation for what Argentines are suffering."

Argentina's Energy Secretariat said in a statement that it expected power to be restored in several hours.

Service stations were forced to close, with the few that had power being inundated with drivers desperate for fuel. ( AP: Tomas F. Cuesta )

Elsewhere in Argentina, several provinces were forced to temporarily delay local elections slated for Sunday.

Argentine state oil company YPF did not immediately respond to requests for comments on how the outage had impacted its operations. Argentina is home to the Vaca Muerta shale formation, one of the world's biggest reserves of shale gas and oil.

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

Shops were forced to close in Buenos Aires on what is usually a busy retail day, Father's Day. ( AP: Tomas F. Cuesta )

Reuters