A massive blackout has left tens of millions of people without electricity in Argentina and Uruguay after an unexplained failure in the neighbouring countries' interconnected power grid.
Authorities were working frantically to restore power, but by mid-afternoon on Sunday about half of Argentina's 40 million people were still in the dark.
Voters cast ballots using the light of their mobile phones in governor elections in the country.
Public transportation stopped, shops closed and patients dependent on home medical equipment were urged to go to hospitals with generators.
"I was just on my way to eat with a friend, but we had to cancel everything. There's no subway, nothing is working," Lucas Acosta, a 24-year-old Buenos Aires resident said.
"What's worse, today is Father's Day. I've just talked to a neighbour and he told me his sons won't be able to meet him."
By mid-afternoon, power had been restored to most of Uruguay's three million people.
But in Argentina, only about 50% of the nation's grid was back up and running, President Mauricio Macri said on Twitter, and officials from the Energy Secretariat were rallying to establish full service to all users.
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 around 7am local time when a key interconnection system collapsed, but the causes were "being investigated and are not yet determined".
Brazilian and Chilean officials said their countries had not been affected.
Officials were not immediately available for comment, but many residents of Argentina and Uruguay said the size of the outage was unprecedented in recent history.
"I've never seen something like this," said Silvio Ubermann, a taxi driver in the Argentine capital.
"Never such a large blackout in the whole country."
Argentine energy company Edesur said on Twitter that the failure originated at an electricity transmission point between the power stations in Yacyreta and Salto Grande in the country's northeast.
Uruguay's energy company UTE said the failure in the Argentine system cut power to all of Uruguay at one point and much of Argentina.
The company said that some Uruguayan coastal cities had service by early afternoon and blamed the collapse on a "flaw in the Argentine network".
Argentina's secretary of energy said the power failed at 7.07am. Only the southernmost province of Tierra del Fuego was unaffected.
"The cause is still under investigation," the energy secretary's office said.
Argentine electric company Edesur said that some 450,000 clients had power restored by 11.53am, with hospitals taking priority.
Uruguayan officials did not provide the number of customers with power back, but a growing list of regions with service indicated that restoration was progressing faster there.