Power has been restored to much of Argentina and Uruguay after a massive electrical failure left tens of millions of people in the dark.
Argentine President Mauricio Macri has promised a full investigation.
Argentine media said the power cut occurred shortly after 07:00 (10:00GMT) on Sunday, causing trains to be halted and failures with traffic signalling.
The blackout was prompted by a failure in an electrical grid that serves both Argentina and Uruguay.
The outage occurred as people in Argentina were preparing to go to the polls for local elections, delaying voting in several regional provinces.
Parts of Paraguay and Chile were also affected, a state energy company said.
What do we know about the blackout?
"A massive failure in the electrical interconnection system left all of Argentina and Uruguay without power," electricity supply company Edesur said in a tweet.
Alejandra Martinez, a spokeswoman for the company, described the power cut as unprecedented.
"This is the first time something like this has happened across the entire country," she said.
Citing official sources, Argentine media reported that the outage was linked to a failure in the transmission of electricity from the Yacycretá hydroelectric dam.
Argentina's Energy Minister Gustavo Lopetegui insisted that the country's electrical system was "very robust," but added that the exact cause of this failure was unclear.
"At the moment we're not ruling out any possibility. But we don't think it is down to a cyber attack," he told reporters.
Power has been restored to more than 80% of customers in Argentina, officials say.
Uruguay's energy company, UTE, said power had been restored to 88% of customers.
How have people been affected?
The combined population of Argentina and Uruguay is about 48 million people.
Among the affected provinces in Argentina were Santa Fe, San Luis, Formosa, La Rioja, Chubut, Cordoba and Mendoza, reports said. Tierra del Fuego in the far south was the only area that remained unaffected because it is not connected to the power grid.
In neighbouring Paraguay, parts of Ayolas, Pilar, Villalbín and the border areas of Misiones and Ñeembucú were also without power.
One of Argentina's biggest water companies, Agua y Saneamientos Argentinos, warned those without power to conserve water, as the distribution of drinking water had been affected by the outage.
Social media reports of the power outage 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. Residents posted pictures of dark towns and cities and long lines of cars queuing at petrol stations.
"Everything came to a halt. Elevators, water pumps, everything. We were left adrift," Juan Borges, who lives in Buenos Aires, told the BBC.
"There are some elderly people on the eighth floor but nothing happened, because the power cut was short. If it had gone on for longer it would have been a whole different story." he said.
Local media have been showing voters casting their local election ballots in the dark, with mobile phones being used as lanterns.