Argentina's main energy company said it was "slowly beginning to restore" electricity after a massive power failure left large swathes of the country and neighboring Uruguay without power.

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

Edesur said on Twitter that power had been returned to 34,000 customers as of Sunday morning.

"The return of electricity generation to the interconnected system of the Federal Capital and Greater Buenos Aires has begun," it said.

It also noted the process would take several hours.

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.