Most of Argentina and Uruguay went without electricity on Sunday after an unexplained failure in the countries' interconnected power system SADI.
The outage, which affected up to 40 million people, interrupted public transport and telecommunication services and cut off water supplies.
The blackout began at around seven o'clock in the morning, just as voters began arriving to polling stations for provincial elections in four Argentine provinces. Voting occurred without major incidents or disruptions, provincial electoral authorities reported.
"This morning we suffered a cut to electric energy across the country due to a failure in the transport system on the coast, and whose causes we have still not been able to determine. We are working to get power back as soon as possible", Argentina's President Mauricio Macri wrote on Twitter.