SAN JUAN, Puerto Rico — A big fire erupted at an electricity plant that powers most of Puerto Rico on Wednesday, causing a blackout that swept across the U.S. territory of 3.5 million people.

The Electric Power Authority said two transmission lines of 230,000 volts each failed. Executive Director Javier Quintana told reporters that he expected most power to be restored by Thursday morning.

Puerto Rico’s fire department said it had extinguished the blaze at the southern power plant, which serves a majority of customers on the island.

Gov. Alejandro Garcia Padilla told reporters the fire began in a switch and initially destroyed a system that provides 30 percent of Puerto Rico’s overall power.

“This is a very serious event,” he said. “The system is not designed to withstand a failure of this magnitude.”

He did not directly answer a question about whether the power company has any kind of backup should another similar situation occur, or if the island was hit by a major hurricane.