By Adam Jourdan, Nicolás Misculin and Eliana Raszewski

BUENOSAIRES (Reuters) – Power had been restored to one-third of Argentina by early afternoon, hours after a massive blackout hit South America on Sunday morning, leaving tens of millions without power, the country’s Energy Secretariat 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 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.

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 called the outage “exceptional” and said it would likely take the rest of the day before power was completely restored. Argentina’s Energy Secretariat said energy distributors nationwide had brought back 5,000 megawatts out of 15,000 MW of total demand.

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.

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

A spokesman for Argentine state oil company YPF told Reuters that all of the plants at its La Plata refinery, a critical link in the country’s oil infrastructure, had been shut down following the outage.

Argentina is home to the Vaca Muerta shale formation, one of the world’s biggest reserves of shale gas and oil. It was not immediately clear to what extent drilling operations there had been impacted.

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

(Reporting by Eliana Raszewski, Nicolas Misculin and Adam Jourdan in Buenos Aires; Additional reporting by Luciano Costa de Paula in Brazil and Rishika Chatterjee in Bengaluru, Daniela Desantis in Paraguay; Writing by Dave Sherwood; Editing by Lisa Shumaker)