efe-epaBuenos Aires

Argentines woke to a massive blackout on Sunday - when the country was both celebrating Father's Day and four provinces were holding elections - but electricity was gradually - and almost fully - restored during the course of the day and authorities have launched an investigation of the disruptive incident.

A "collapse" in the Argentine Interconnection System (SADI) caused the "massive outage in electric energy" across Argentina and Uruguay, the Energy Secretariat said Sunday.

Shortly after 7 am on a day bringing intense rain over a large part of the country, homes and streets were left in complete darkness and it was not until three hours later that the situation "slowly" began to become alleviated with - by 6 pm - some 89 percent of users having regained their electric power.

The sudden blackout, which also affected Uruguay and parts of Paraguay - resulted in trains and the Buenos Aires metro suspending service, although the country's main airports were able to continue providing service using their own generators.

The blackout also affected potable water distribution, although the company responsible for water supply in the capital and the Greater Buenos Aires Area - where 13 million of the country's 40 million people live - asked users to scale back water consumption in their homes.

Regarding hospitals, in the capital an emergency plan was implemented to guarantee that patients could get medical care, including in intensive care facilities and surgical theaters.

In several Twitter messages posted seven hours after the start of the blackout, President Mauricio Macri called the incident "unprecedented" and said that it would be thoroughly investigated.

Energy Minister Gustavo Lopetegui called a press conference to say that the blackout was "very serious" and must not be allowed to happen again.

Lopetegui also said that, although the crisis was serious and the government was not ruling out anything, "I would say that we don't believe this was a cyberattack. It's not among the main alternatives we're considering."

He added that, basically, Argentina's electric power grid was "very robust."

The official explanation for the power outage was the collapse of the SADI electric power system, specifically an electricity distribution linkage in northeastern Argentina near the border between the hydroelectric plants at Yacyreta and Santo Grande, which are Argentine-Paraguayan and Argentine-Uruguayan managed, respectively.

In the coming 48 hours, the companies responsible for electricity distribution will have to present preliminary reports on what happened with a complete report to come within 10 days, and it is expected, according to law, that sanctions will be levied against those found to be responsible.

Disinformation and uncertainty marked the start of the day, although because it occurred on a Sunday it was not as disruptive as might have been the case had it happened on a weekday, when millions of people would have been trying to get to work, although many businesses and restaurants catering to Father's Day customers did have to close and sustain significant losses of potential revenue.

In particular, the provinces of Santa Fe, San Luis and Formosa - where gubernatorial elections were being held - were affected, but those votes were not cancelled. Nevertheless, problems were experienced at polling places due to lack of lighting, especially early in the morning.

Elections were also being held on Argentina's southernmost province, Tierra del Fuego, but that was the only province that did still have electric power, since it does not depend on the same power grid as the rest of the affected area.

The historic blackout came just a little over four months before the presidential election scheduled for Oct. 27, to be preceded by primaries on Aug. 11, and the main opposition candidate was harsh in his criticism of the government.

Frente de Todos presidential candidate Alberto Fernandez, whose running mate is former President Cristina Fernandez de Kirchner, mentioned the increases in electricity rates pushed by the government in recent years, noting that "Millions of Argentines who have had to pay monthly amounts ... that benefit the friends of power, are still waiting to have power restored to their homes," he stressed on Twitter

Macri, since he came into office in December 2015, has pursued a policy of utility rate increases with the aim - the government has said - of normalizing the country's power system after years under Fernandez de Kirchner during which investment there was neglected.

However, the rate hikes have sparked huge controversy, and the opposition has accused the government of favoring the energy magnates with its policies.

Earlier in the day, electricity company Edesur said on social media that its control center had started the work of "normalization and slowly started restoring electric energy service to the grid," which was providing electricity to just "the first 34,000 customers."

"It's going to take some hours, because in addition to the cause still not being clear, getting the generation and transmission back online so that it reaches every home will take several hours," Edesur spokeswoman Alejandra Martinez told Radio Mitre.

Uruguayan state-owned electric utility (UTE) said that parts of the country were affected by the outage in Argentina, although UTE spokesman Ariel Ferragut told EFE that the utility company was "slowly restoring" electric service across "all of the country."

The Paraguayan National Electricity Administration (ANDE) said in a statement that the outage in Argentina and Uruguay caused "the blockage of seven generating units at the Yacyreta Hydroelectric Complex."

The electric utilities operating in southern Brazil, meanwhile, had said that the blackout had not affected them, with Companhia Estadual de Energia Eletrica (CEEE) and Rio Grande Energia, which serve all of southern Brazil, saying that they were operating normally.