Power has almost all been restored to millions of homes after a "massive" blackout which affected the whole of Argentina and Uruguay.
Edusur, a major provider to the two nations, confirmed the outage in a tweet on Sunday morning and engineers had been working around the clock to restore power.
Argentinian website Infobae said Edusar had said "the blackout is at a country level and also affects Uruguay", adding that nothing like this had ever happened.
The outage is understood to have happened at 7am local time, about 12pm GMT, and affected millions of people.
By evening, electricity had returned to 98% of Argentina, according to state news agency Telam.
Power had also been restored to most of Uruguay's three million people.
Argentina's power grid is generally known for being in a state of disrepair, with substations and cables insufficiently upgraded as power rates remained largely frozen for years.
The country's energy secretary Gustavo Lopetegui said the blackout occurred when a key Argentinian inter-connection system collapsed.
Edesur said on Twitter that the failure originated at an electricity transmission point between the power stations at the country's Yacyret dam and Salto Grande in the country's northeast.
But why it occurred was still unknown.
Silvio Ubermann, who lives in Buenos Aires, said: "If this had been a weekday, it would've been chaos. Sometimes there is not light in the summertime as a result of high electricity consumption, but never such a large blackout in the whole country.
"I've never seen something like this."
People reported blackouts in several regions. One twitter user shared an image of the darkened Mar del Plata.
It also impacted regional elections taking place in several parts of Argentina.
Images on local media showed some people casting their votes while others hold their mobile phones above them to cast light on the ballot paper.
There were reports that some places opened later for voting because of the power outage.