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The Latest on the blackout in Argentina and Uruguay (all times local):

2:45 p.m.

Argentine energy company Edesur says that electricity has been restored to more than 1.5 million people.

President Mauricio Macri also said that 50% of the country had power after a massive failure in Argentina and Uruguay on Sunday morning left many in the dark.

Macri said on Twitter: "As time passes, service will be restored for all customers."

The cause of the blackout was not immediately clear.

Edesur said that a failure in the Argentine interconnection system originated at an electricity transmission point between the power stations of Yacyretá and Salto Grande in the northeastern part of the country.

Uruguayan energy company UTE said 75% of service had been restored in its country.

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2 p.m.

A massive blackout left millions without electricity in Argentina and Uruguay on Sunday after an unexplained failure in the neighboring countries' interconnected power grid. Authorities were working frantically to restore power but only about a half a million in Argentina had electricity back by early afternoon.

Voters cast ballots by the light of cell phones 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.

In Uruguay, power was being more steadily restored, with lights back on in at least three regions by early afternoon.