Social media reports of the power were widespread - from the capital Buenos Aires in the north, to Mendoza in the west and Comodoro Rivadavia in the south, among many other cities.
It added that the system went down at 7:06 am local time.
The Argentinian company later said it had started the "normalization process, which will require several hours".
A major power outage has left parts of Argentina and neighbouring Uruguay in the dark (Getty) A sign, which usually displays the price of various types of gas electronically, stands without power during a blackout, in Buenos Aires. A "technical issue" or simple humidity could have triggered the breakdown, said Carlos Garcia Pereira, head of Transener, Argentina's largest power-transmission operator.
Around 48 million people in are affected by the outage.
The power outage fell on a day of provincial elections in some of Argentina's provinces.
Paraguay's National Electricity Administration said the breakdown began with an as yet unexplained event in Argentina's power network, which led to a fault that disconnected all generators at the Yacyreta hydroelectric dam from the regional grid on Sunday morning.
Several Argentine provinces were forced to temporarily delay local elections.
A vendor waits for customers during a nationwide blackout, in Buenos Aires, Argentina, on Sunday, June 16, 2019.
Argentina and Uruguay were scrambling to restore power to millions of people after a massive power failure hit the Argentine grid early on Sunday, Argentina's Energy Secretariat said.
In Buenos Aires, the Constitucion railway station was empty, with trains halted, Bloomberg reported.
The cause of the blackout was not immediately clear.
"Some coastal cities already have service and work continues toward general restoration (of power)", it said.
Since taking office, Argentina's President Mauricio Macri has said that gradual austerity measures were needed to revive the country's struggling economy.
"I've never seen something like this", Ubermann said. He has cut red tape, and tried to reduce the government's budget deficit by ordering job cuts and reducing utility subsidies, which he maintained was necessary to recuperate lost revenue due to years-long mismanagement of the electricity sector.
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