A massive blackout left tens of millions of people without electricity in Argentina, Uruguay and Paraguay on Sunday in what the Argentine president called an "unprecedented" failure in the countries' power grid.
Authorities were working frantically to restore power, and by the evening electricity had returned to 98 per cent of Argentina, according to state news agency Telam. Power also had been restored to most of Uruguay's three million people as well as to people in neighboring Paraguay.
On Sunday morning, Argentine voters were forced to cast ballots by the light of cellphones in gubernatorial elections. Public transportation was halted, shops closed and patients dependent on home medical equipment were urged to go to hospitals with generators.
"This is an unprecedented case that will be investigated thoroughly," Argentine President Mauricio Macri said on Twitter.
Argentina's power grid is generally known for being in a state of disrepair, with substations and cables that were insufficiently upgraded as power rates remained largely frozen for years.
The country's energy secretary said the blackout occurred at about 7am local time (2pm UAE) when a key Argentine interconnection system collapsed. By mid-afternoon nearly half of Argentina's 44 million people were still in the dark.
The Argentine energy company Edesur said on Twitter that the failure originated at an electricity transmission point between the power stations at the country's Yacyretá dam and Salto Grande in the country's northeast.
But why it occurred was still unknown.
An Argentine independent energy expert said that systemic operational and design errors played a role in the power grid's collapse.