Cheers erupted as lights slowly began to flicker across Puerto Rico overnight as it struggled to emerge from an island-wide blackout following a fire at a power plant that caused the aging utility grid to fail.

More than 390,000 of 1.5 million homes and businesses served by the power utility had electricity restored by late Thursday, with cries of, "The lights are back on!" echoing through some neighborhoods.

Governor Alejandro Garcia Padilla said he expected more than half of customers to have power by Friday morning.