Cars drive through an unlit street during a blackout in Buenos Aires, Argentina, on June 16, 2019. A massive blackout left tens of millions of people without electricity in Argentina, Uruguay and Paraguay on Sunday. Photo: AP Photo/Tomas F. Cuesta.

Buenos Aires - Suppliers in Argentina and Uruqyay said late Sunday that power has been almost completely restored in the two countries, roughly a day after a massive blackout left millions in South America without electricity.

Some 47 million people in both countries mostly all have power again, the largest Argentine energy providers Edesur and Edenor and their Uruguayan counterpart UTE said.

The two countries had been affected by the massive blackout for up to 15 hours.

Edesur said that the blackout started at 7:07am (1007 GMT) Sunday, some 50 minutes before sunrise. Neighbouring Uruguayan energy company UTE said its outage started a minute earlier and had left the entire country without service.

The system was shut down automatically because a voltage destabilization was detected, Argentina's Energy Secretary Gustavo Lopetegui said at a press conference.

On a Sunday without extreme temperatures, the grid has a power reserve of 20 per cent, so the complete shutdown was not normal, he said.

Lopetegui did not rule out a cyberattack, but did not consider it to be the leading hypothesis. A result of the ongoing investigations will be known only in seven to 10 days, he said.

In the Greater Buenos Aires area electricity was restored after a few hours, and eight hours after the blackout started, 56 per cent of customers in Argentina were reconnected to the grid, Lopetegui said.

The blackout hit during Argentina's winter, when the country relies on heating, and one witness named Sara told dpa it was cold on the Argentinian coast south of Buenos Aires at the time of the outage.

Traffic lights were not working, and it was also not possible to refuel cars. "It's very strange, it's never been this big," the woman said.

The outage comes as Argentina is suffering from a severe economic crisis, and also as four provinces were staging local elections.

With a largely dilapidated infrastructure, Argentinians are used to frequent isolated power outages, but more typically in the summer when air-conditioning systems overload the network.

Parts of Brazil, Chile and Paraguay were also affected by the blackout.

The Misiones region in southern Paraguay could be reconnected after 40 minutes after it was switched to the Brazilian hydropower plant at Itaipu.

Chile's Radio Cooperativa reported that the power outage there had occurred and was fixed before Argentina's.

Uruguay and Argentina are connected via the joint Salto Grande power plant on the Uruguay River, 350 kilometres north of Buenos Aires, while Paraguay shares the Yacireta power plant on the Parana River with Argentina.

UTE later said service had been restored in parts of Uruguay's coastal cities and the greater Montevideo area, while work was continuing for a full restoration.

Water supply in Argentina was also impacted by the outage, with local supplier Aguas Santafesinas urging people to restrict their use of drinking water to when absolutely essential.

Underground rail transport in the Argentinian capital was also brought to a standstill and Buenos Aires Metro Metrovias said all six of its lines were without service during the outage.

Locals in Uruguay and Argentina took to social media using the hashtag #SinLuz (no lights) after waking up without power.