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 Argentinas 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.
Lucas Acosta, 24, who lives in Buenos Aires resident said: I was just on my way to eat with a friend, but we had to cancel everything. Theres no subway, nothing is working.
Whats worse, today is Fathers Day. Ive just talked to a neighbour and he told me his sons wont be able to meet him.
Locals were forced to sleep on sidewalks to escape the heat.
By mid-afternoon, power had been restored to most of Uruguays three million people.
But in Argentina, only about 50 per cent of the nations 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.
Argentinas 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 countrys 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.