The failure began shortly after 7am on Sunday, leaving officials scrambling to restore electricity. Much of the network had been restored by Monday morning, as Argentinia's president Mauricio Macri promised a full investigation into what caused the blackout.
The power cut crippled public transportation in Buenos Aires, Argentina’s capital, and in other major cities.
It also cut off the water supply, closed shops and brought down phone and internet communications across the country.
“A massive failure...in the system left Argentina and Uruguay without power,” a spokesperson for energy company Edesur said in a statement. Edesur has 2.5m customers, according to its website.
“If this had been a weekday, it would’ve been chaos,” said Silvio Ubermann, a Buenos Aires resident.
“Sometimes there is not light in the summertime as a result of high electricity consumption, but never such a large blackout in the whole country.
“I have never seen something like this.”
Edenor, Argentina’s largest energy company, also tweeted about the failure, as millions woke to the blackout.
“Due to a general failure in the interconnection system, Argentina and neighbouring countries are without electricity,” the energy distributor said.
Several Argentine provinces were forced to temporarily delay local elections due to the blackout.
The cause of the outage was still unclear as of Monday morning, but Argentina’s energy agency confirmed in a statement it had begun an investigation.
EPEC, another energy company, said the outage had affected most of the country, including Córdoba and Santa Fe, as well as Buenos Aires.
UTE, Uruguay’s electricity body, confirmed that the neighbouring country was affected in a statement published on Twitter.
“At 7.06 a defect in the Argentine network affected the interconnected system leaving the entire national territory without service,” the organisation said.
UTE said work was underway to restore power. The company added that electricity had been restored to some cities on Uruguay’s coast.
More than 48m people live across Argentina and Uruguay.
Users on social media, claiming to be resident in the countries, shared photographs of the blackout.
Since taking office, Argentina‘s president Mauricio Macri has said that gradual austerity measures were needed to revive the country’s struggling economy.
He has cut red tape and tried to reduce the government’s budget deficit by ordering job cuts and cutting utility subsidies.
A spokesman for Brazil’s power system operator (ONS) said the outage had not impacted the country.
Additional reporting by agencies