Millions of people across Argentina and Uruguay were left without power early on Sunday after a massive power failure hit the Argentine grid.
The cause of the outage was still unclear, but the Argentina energy agency said in a statement that authorities were working to restore power to provinces in the northwestern corner of the country.
Several major cities, including the Argentine capital of Buenos Aires, awoke to darkness on Sunday morning, with public transportation brought to a halt by the outage. Water supplies were also affected while phone and internet communications went down.
A spokesman for Brazil's power system operator (ONS) said the outage had not impacted the regional neighbor to the north.
Argentina's main energy company has said it is "slowly beginning to restore" electricity after the massive power failure left large swathes of the South American country and neighbouring Uruguay in the dark.
Writing on Twitter , Edesur said power had been returned to 34,000 customers as of Sunday morning.
The company said: "The return of electricity generation to the interconnected system of the Federal Capital and Greater Buenos Aires has begun."
It 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.
It added that the blackout was due to a "flaw in the Argentine network" which left the entire country and several regions of Uruguay without electricity.
Argentina's energy secretary said the blackout happened around 7am local time, when a key interconnection system collapsed.
It said the causes "are being investigated and are not yet determined".
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.
