Argentina is investigating what caused a massive power cut that hit nearly 50 million people.

President Mauricio Macri called the outage "unprecedented", promising a thorough inquiry.

Almost all of Argentina, neighbouring Uruguay and some areas of Paraguay woke up to darkness on Sunday.

The disruption hit public transport while hospitals were forced to run on generators. Power had largely been restored by the end of the day.

Argentine officials said it began with a failure in the country's "interconnection system" but that the exact cause was unknown.

Energy Minister Gustavo Lopetegui said it was unlikely that a cyber-attack was responsible, and that it would take 10-15 days for the results of the investigation to be published.

Despite the massive outage he said the country's electrical network was "very robust".

How bad was it?

The power cut hit almost all of Argentina's 44 million population, with just one province not connected to the main system spared. Uruguay's 3.4 million inhabitants were also affected. The two countries share a power grid.

Local elections still took place as scheduled in Argentina, with local media showing voters using their mobile phones for torches.

Restaurants that expected to be full for Father's Day instead remained closed.

Residents posted pictures of dark towns and cities and long lines of cars queuing at petrol stations.

"Everything came to a halt. Elevators, water pumps, everything. We were left adrift," Juan Borges, who lives in Buenos Aires, told the BBC.

"There are some elderly people on the eighth floor but nothing happened, because the power cut was short. If it had gone on for longer it would have been a whole different story." he said.

A similar blackout hit Brazil in 2009, with a massive blackout leaving about 60 million without power.