SAN JUAN — Most of Puerto Rico was without electricity Thursday following a fire at a power plant that knocked out the entire grid, leaving most of the island’s 3.5 million people without service.

The vast majority of Puerto Ricans who don’t have generators were forced to spend the night in darkness and without air conditioning in the tropical heat. They awoke to find most businesses and public offices closed, with the government saying it could be 24 hours before power would be restored.

But telecommunications services are working normally and the Luis Muñoz Marin International Airport continues to operate, Alejandro García Padilla, the governor of Puerto Rico, said at a news conference.

“Puerto Rico is not prepared for something like this,” said 23-year-old Celestino Ayala Santiago, who slept in his car so he could have some air conditioning to escape the heat.

The roar of generators and the smell of diesel fuel filled the morning air, as the temperature rose to around 90 degrees. There were long lines at fast food restaurants that had power, but the traffic was light, with most people staying at home.

Javier Quintana, executive director of the Electric Power Authority, said power was restored to more than 130,000 of the total of 1.5 million customers by early Thursday morning and that it may be back to normal on Friday.

Gov. Alejandro Garcia Padilla warned it would be a slow process. “Given that the system is so old, numerous setbacks could occur,” the governor said. “The system is not designed to withstand a failure of this magnitude.”

Authorities said Wednesday’s outage caused 15 fires across Puerto Rico as a result of malfunctioning generators, including at the upscale Vanderbilt Hotel in the popular tourist area of Condado and at the mayor’s office in the northern coastal town of Catano. All those fires were extinguished and no one was injured, officials said.