SAN JUAN, Puerto Rico — Cheers erupted as lights slowly began to flicker back on across Puerto Rico overnight as the U.S. territory struggled to emerge from an island-wide blackout following a fire at a power plant that caused the aging utility grid to fail.

About 75 percent of 1.5 million homes and businesses served by the power utility had electricity restored by Friday morning, with cries of, "The lights are back on!" echoing through some neighborhoods.

"We've surpassed our expectations," Gov. Alejandro Garcia Padilla said. "We're working hard to restore power to our other customers ... I understand the annoyance of being without electricity."

He anticipated that the majority of Puerto Ricans will likely have power by Saturday, more than 72 hours after the island-wide outage.

The lights went out briefly during Friday's press conference at the island's emergency operations center, provoking a laugh from government officials addressing reporters. Garcia said temporary power outages would still occur in upcoming days given the growing demand for electricity as more and more customers became reconnected.

The blackout hit the entire island of 3.5 million people early Wednesday afternoon and prompted Garcia to activate the National Guard and declare a state of emergency. Public schools remained closed on Friday, and heavy storms that hit the island Thursday afternoon knocked out power to some areas where electricity had been restored. Meanwhile, some 205,000 people remained without water.

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. Most Puerto Ricans don't have generators, and many once again dragged mattresses out to balconies and porches to spend the night outside.

"It's been horrible," said San Juan resident Elizabeth Maldonado, adding that she was resigned to another sleepless night. "I take showers every three hours at night to stay refreshed."

For those who could afford it, hotels offered special rates for residents that were quickly snapped up.

During the outage, long lines formed at ice plants, supermarkets and gas stations. Elsewhere, people crouched around power outlets at generator-powered supermarkets and malls to charge cellphones.

The governor said at least one person died the first night from exposure to carbon monoxide after setting up a personal generator. A 76-year-old man was taken to the hospital in good condition after spending the night trapped in an elevator at a government building, Garcia said. In addition, four police officers were struck by vehicles hit while directing traffic but were expected to recover.

Localized power outages are common in Puerto Rico, which has an outdated energy infrastructure, but widespread failures such as this are extremely rare.

The Electric Power Authority said it was trying to determine what caused the fire at the Aguirre power plant in the southern town of Salinas. The fire apparently knocked out two transmission lines that serve the broader grid, which tripped circuit breakers that automatically shut down the flow of power as a preventive measure, officials said. Executive director Javier Quintana said a preliminary investigation suggests that an apparent failure on one transmission line that might have been caused by lightning caused the switch to explode.

Garcia rejected suggestions the blackout was caused by maintenance problems that have plagued the utility for years, largely a result of the island's economic and fiscal crisis. He said the switch where the fire happened had been properly maintained.

It was not yet clear how much damage the fire caused. The utility is struggling with a $9 billion debt that it hopes to restructure as it faces numerous corruption allegations. Company officials have said they are seeking revenue to update outdated equipment.

Danica Coto on Twitter: www.twitter.com/danicacoto