Argentina and Uruguay have been left without power after a "failure in the electrical interconnection system," Trend reports citing Sky News.
Edusur, a major provider to the two nations, confirmed the outage in a tweet.
The outage is affecting about 48 million people.
Argentine website Infobae said Edusar confirmed "the blackout is at a country level and also affects Uruguay" and said nothing like this had ever happened.
The outage is understood to have happened at 7am local time, about 12pm GMT.
People have reported blackouts in several regions. One twitter user shared an image of the darkened Mar del Plata.
The power outage means trains, subways and the metro cannot run and could affect whether schools and offices can open today.
It could also impact the regional elections taking place in several parts of Argentina.
Infobae also reports that the website for the National Entity for Energy Regulation (ENRE) is down. The site should report power outages in real time.