A massive electrical failure has left almost all of Argentina and Uruguay without power, according to a major Argentine electricity provider.
Parts of Paraguay were also affected, a state energy company said.
Argentine media said the power cut occurred shortly after 07:00 (11:00 BST), causing trains to be halted and failures with traffic signalling.
It came as people in parts of Argentina were preparing to go to the polls for local elections.
"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."
Argentina's energy secretary, Gustavo Lopetegui, said the cause of the power failure had not yet been determined. The Ministry of Civil Protection estimated that parts of the service could be restored in about seven or eight hours.
Uruguay's energy company, UTE, said in a series of tweets that power had been restored to coastal areas and to areas north of Rio Negro.
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 is the only area that remains unaffected because it is not connected to the power grid.
Paraguay's National Administration of Electricity (ANDE), said in a tweet that 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 limit their water use, as the distribution of drinking water had been affected by the power cut.
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. Residents posted pictures of dark towns and cities using the hashtag #SinLuz.