Tens of millions lost their power in Argentina and Uruguay on Sunday as Argentinian president promises investigation.

A massive blackout left more than 44 million people without electricity in Argentina and Uruguay on Sunday after an unexplained failure in the countries’ interconnected power grid.

Authorities were scrambling to restore power but only around half a million people in Argentina had electricity back by the early afternoon local time.

“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 told the Associated Press news agency.

“What’s worse, today is Father’s Day. I’ve just talked to a neighbor and he told me his sons won’t be able to meet him.”

Argentinian President Mauricio Macri said the blackout was “unprecedented” and pledged a thorough investigation.

He said the outage had been caused by a failure in the country’s coastal grid, but said that officials did not know what caused the problem.

In Uruguay, power was being more steadily restored, with lights back on in at least three regions by early afternoon. Officials said they expected most of the country of three million people to have power back soon.

Argentina’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.”

Energy company Edesur Argentina said it had returned service to 450,000 of its clients in the greater Buenos Aires area by noon local time (1600 GMT), but called the outage “exceptional” and said it would likely take the rest of the day before power was completely restored.

190613202509099

“The return of electricity generation to the interconnected system of the Federal Capital and Greater Buenos Aires has begun,” Edesur said, noting the process would take several hours.

The massive blackout on Father’s Day left the Argentinian capital Buenos Aires dark early this morning, hobbling public transportation, cutting off water supply and crippling phone and internet communications across the city.

Images from social media showed long lines of cars at the few service stations still in operation in Argentina’s capital city and traffic lights dark, creating chaos in places even on a normally quiet Sunday.

Argentina and Uruguay have a common power grid centred on the bi-national Salto Grande Dam, 450km north of Buenos Aires. Another Argentinian power company confirmed that the failure knocked out electricity throughout Argentina, without specifying the cause.

Several Argentine provinces were forced to temporarily delay local elections as a result of the blackout.

Reporting from Buenos Aires, Al Jazeera’s Teresa Bo said it was a chaotic morning in the Argentinian capital.

“Streetlights were completely off, public transport was down, all that combined with a major storm,” she said.

“The government is saying that the main cause of the outage was generated in a dam in northeast Argentina, right at the border with Paraguay. They say that a problem with one of the lines there is what caused this.

However, it has not been confirmed. It is expected that the secretary of energy will give a press conference later today.”

Social media reports on the blackout were widespread.

“Huge blackout in Argentina: the City, the Province of Buenos Aires and Santa Fe were left in the dark,” a news agency posted on Twitter.

Gigantesco apagón en la Argentina: la Ciudad, la Provincia de Buenos Aires y Santa Fe se quedaron a oscuras https://t.co/nSOEYiQ5gg pic.twitter.com/gBlDCfDWI5 — TN – Todo Noticias (@todonoticias) June 16, 2019

“What’s happening with this blackout,” a user on Twitter asked the government.

“Never in my life have I seen such a gigantic power cut,” another user wrote.

https://twitter.com/fernanmontiel/status/1140202369243844609?ref_src=twsrc^tfw