"This is an unprecedented case, which will be thoroughly investigated," Macri tweeted.
Lights went out in all of Argentina and neighbouring Uruguay at around 7 a.m. on Sunday. Utility companies in both countries blamed a technical malfunction. Cities in Brazil, Chile and Paraguay also experienced outages.
Argentine power distributor Edesur said a failure in the interconnection system on the stretch between the Yacyreta and Salto Grande dams on the Argentine coast knocked out the power.
Electricity supply has been restored to 2 million homes, or 80 per cent of their Argentine clients, the Buenos Aires-based company added. Uruguay's UTE state power company said 25 per cent of their clients were still without power.
In March major power outages took place in Venezuela, affecting 21 out of the country's 23 states. The authorities blamed the blackout on what they called was a "sabotage" at the Guri hydroelectric power plant, which generates electricity for almost the whole country.