Garcia Padilla said that the power failure on Wednesday is a sign that the USA commonwealth needs to invest its resources in priority issues for the public, such as improving the electric infrastructure, and leave other matters - such as paying off its huge public debt - for a later time.

Government leaders are saying it could be another 24 hours or more before all power is restored.

The governor of Puerto Rico has said power is slowly being restored almost 24 hours after a blackout swept across the island.

That still left about a third of the population without power.

While those with power celebrated a return to normalcy, others lamented having spent another night in darkness with no air conditioning in the tropical heat.

On Thursday, many businesses and public offices were closed.

Investigators are still trying to determine what caused the fire. The cause of the fire is not yet known.

"This is a very serious event", Puerto Rico Gov. Alejandro Garcia Padilla said of the outage.

As sunset approached, lines formed at ice plants, supermarkets and gas stations, and people crouched around power outlets at generator-powered supermarkets and malls to charge cellphones. Many Puerto Ricans dragged mattresses out to balconies and porches to spend the night outside, doing what they could to ward off mosquitoes in the still air. "The system is not created to withstand a failure of this magnitude".

As of Thursday morning, service had been restored to just 130,000 customers and, barring any mishaps, more than half of the island could have power by the afternoon, the company said via Twitter.

"It's taking way too long", said 74-year-old Magdalena Concepcion as she waited at a bus stop.

Almost everyone on the island of 3.5 million people lost power.

"To see everything blacked out, my God", said Virginia Davila, a nurse's assistant who lives on the 11th floor of an apartment building in San Juan. He said the switch where the fire began had received proper maintenance.

The electricity provider is in the process of restructuring $9 billion of debt. Some people opted to not go home and hotels in the capital of San Juan quickly filled up.

The power outage angered many Puerto Ricans who on average, pay power bills that are twice as costly as bills on the USA mainland, according to the Orlando Sentinel. Company officials have said they are seeking more revenue to update what they say is outdated equipment.

As the sun set, people crowded into restaurants running on generators.

The blackout knocked out traffic lights, snarling the island's roads.