BUENOS AIRES, Argentina (AP) — A massive blackout left more than 44 million people without electricity in Argentina and Uruguay yesterday, after an unexplained failure in the neighbouring countries’ interconnected power grid.

Authorities were working frantically to restore power but only about a half a million in Argentina had electricity restored by early afternoon.

Voters cast ballots by the light of cellphones in gubernatorial elections in Argentina. Public transportation halted, shops closed, and patients dependent on home medical equipment were urged to go to hospitals with generators.

“I was just on my way to eat with a friend, but we had to cancel everything. There’s no subway; nothing is working,” said Lucas Acosta, a 24-year-old Buenos Aires resident. “What’s worse, today is Father’s Day. I’ve just talked to a neighbour and he told me his sons won’t be able to meet him.”

In Uruguay power was being more steadily restored, with lights back on in at least three regions by early afternoon. Officials said they expected most of the country, of three million people, to have light restored by today.

Argentina’s power grid is generally known for being in a state of disrepair, with substations and cables that were insufficiently upgraded as power rates remained largely frozen for years.

The country’s energy secretary said the blackout occurred around 7 am local time when a key interconnection system collapsed, but the causes were “being investigated and are not yet determined”.

Brazilian and Chilean officials said their countries had not been affected.

Officials were not immediately available for comment, but many residents of Argentina and Uruguay said the size of the outage was unprecedented in recent history.

“I’ve never seen something like this” said Silvio Ubermann, a taxi driver in the Argentine capital. “Never seen such a large blackout in the whole country.”

Argentine energy company Edesur said on Twitter that it was “slowly beginning to restore” electricity, and power had been returned to 290,000 customers as of Sunday morning, at least some of whom were in the capital.

It said the failure originated at an electricity transmission point between the power stations in Yacyretá and Salto Grande on the Argentine coast.

Uruguayan energy company UTE said the failure in the Argentine system cut power to all of Uruguay at one point and much of Argentina. The company said that some Uruguayan coastal cities had service by early afternoon, and blamed the collapse on a “flaw in the Argentine network”.