BUENOS AIRES (Reuters) -- Power began to trickle back into the grid by midday after a massive blackout hit Argentina, Uruguay and Paraguay early on Sunday, leaving tens of millions without power, authorities said.
Argentina’s grid "collapsed” around 7 a.m. local time (1100 GMT), leaving the entire country without power, Argentina´s Energy Secretariat said in a statement. The outage also cut electricity to swaths of neighboring 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.
Argentina’s energy agency said in a statement it had begun investigating the causes of the outage, but had not provided further details by midday.
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 (1600 GMT), 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.
The massive blackout on Father’s Day 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. Stores aren’t open. It spoiled Father’s Day,” said 75-year-old retiree Liliana Comis, of Buenos Aires.
The outage also spilled over into local and national politics.
Alberto Fernandez, a Peronist presidential candidate looking to unseat incumbent Mauricio Macri in this year’s hotly contested election, tweeted that "the President should...give an explanation for what Argentines are suffering.”
Macri had yet to make a statement on social media as of late morning in Argentina.
Argentina’s Energy Secretariat said in a statement mid-morning on Sunday that it expected power to be restored in several hours.
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 (ONS) said the outage had not impacted the regional neighbor to the north.