Argentina and Uruguay were working frantically to return power on Sunday, after a massive power failure left large swaths of the South American countries in the dark.

It was unclear how many people had been affected by the blackout, but the nations have a combined population of more than 45 million.

Argentina's energy secretary said that the blackout occurred around 7 a.m. local time when a key interconnection system collapsed, but the causes were "being investigated and are not yet determined."

"If this had been a weekday, it would've been chaos," said Silvio Ubermann, a resident of the capital Buenos Aires. "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've never seen something like this," Ubermann added.

Argentine energy company Edesur said on Twitter that it was "slowly beginning to restore" electricity, and power had been returned to 290,000 customers as of Sunday morning, at least some of whom were in the capital. It noted the process would take several hours due to the seriousness of the situation.

Uruguayan energy company UTE tweeted that the system was being reinstated from scratch.

"There are already coastal cities with service and work continues toward general restoration," it said.

It said the blackout was due to a "flaw in the Argentine network" which left the entire country and several regions of Uruguay without electricity.

On Sunday, large cities were paralyzed as shops closed and publication transportation ground to a halt. Many appeared to be spending the rainy Father's Day inside.

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.