A "massive failure" in the interconnected electrical grid left Argentina, Paraguay and Uruguay completely without power Sunday.
Parts of Chile and southern Brazil experienced outages as well, the Buenos Aires-based utility distributor Edesur said.
Authorities were working frantically to restore power, but by mid-afternoon nearly half of Argentina's 44 million people were still in the dark.
Uruguay's energy company UTE said the failure in the Argentine system cut power to all of Uruguay at one point and blamed the collapse on a "flaw in the Argentine network.''
Edesur said the priority was to restore power to hospitals and health centers first.
Public transportation and business ground to a halt. In Argentina, people voted in gubernatorial elections, using cell phones to light their ballots.
"This is an extraordinary event that should have never happened,'' Argentine Energy Minister Gustavo Lopetegui said.
He said the cause of the disruption was being investigated and that workers were working to restore electricity nationwide by the end of the day.
Edesur said the failure began at an electricity transmission point in the country's northeast at about 7 a.m. local time.