A massive electrical failure on Sunday left all of Argentina and Uruguay without power, according to a major Argentine electricity provider.
Reports said the power cut had also affected parts of Brazil and Paraguay.
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 supplier company Edesur said in a tweet.
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.
Edesur said that power had been restored to parts of Buenos Aires and local media reported that two airports were operating on generators in the capital.
Uruguay's energy company, UTE, said in a tweet that power had been restored to coastal areas.
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 is the only area that remains unaffected.
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.
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.
Residents posted pictures of dark towns and cities using the hashtag #SinLuz.