As New Yorkers sweltered during the fourth day of a heat wave and city officials urged residents to conserve energy, the air was extra cool and the lights burned bright in at least two city-run buildings.

No hearings were scheduled, but more than a dozen lightbulbs glowed in the empty City Council chambers, while chandeliers twinkled in the hallways of City Hall even while the sunlight was pouring through the windows.

In one corner of the building, a seven-bulb chandelier illuminated a locked door, an empty phone booth, and a sign from the conservation office of the Department of Citywide Administrative Services that began: “To reduce the risk of power interruption, please make every effort to reduce your use of electrical power. Turn off all unnecessary lights and equipment.”

A wall thermostat carried into the building registered 74 degrees, four degrees lower than the 78 recommended by the city.

At police headquarters, a brightly lit but vacant auditorium was 75 degrees, according to the Taylor-brand wall thermostat, which registered 100 degrees outside. No events were scheduled in the auditorium yesterday.

At a news conference, Mayor Bloomberg told New Yorkers to “turn out lights wherever possible.”

“Don’t run air conditioners in a room when you’re not there. We can get through this fine,” he said. “Just turn down the thermostats.”

More than 15,000 New York electricity customers — a customer can represent an entire apartment building — have lost power at some point since Sunday as the temperature and electricity use has reached record highs for June, the city’s utility, Consolidated Edison, reported yesterday.

A spokesman for DCAS, Mark Daly, said his agency reminds other city agencies of the summer energy guidelines on peak days such as yesterday.

“We also recognize that facilities managers have to exercise common sense,” he said.

A spokesman for the mayor, Jason Post, said some air-conditioning and “over half the lights” had been turned off inside City Hall, and noted that most of the building was cooled with a steam system that uses less energy than electric air conditioners. The Municipal Building and the Department of Education headquarters in Tweed Courthouse — where the temperature was 78 — use the same system.

A police department spokesman, Assistant Chief Michael Collins, said the thermostat at headquarters was set at 78. He said the auditorium was lit because maintenance men had been doing work there.

Some New Yorkers said they were seeking to follow the mayor’s advice. Lynn Baas, 42, who works for the New York County clerk, said she had taken it upon herself to shut off lights in bathrooms and hallways at her office building over the past few days. She also said that because of her concern for the environment, she doesn’t even own an air conditioner.

Stephanie Chalmers, 29, who works for a brokerage firm, said she flipped her lights off when the temperature began to rise.

“I try my best to be energy-conscious in this hot weather,” she said.

Mr. Bloomberg said he would not close schools, many of which lack air-conditioning in every classroom, in reaction to the extreme heat.

“This is going to come as a very big surprise to you, but people of my generation went to schools without air conditioners,” the mayor said. “The schools are being very careful to make sure that safety is being maintained in the classroom. They can open the windows, there’s a breeze.”

The teachers union filed a complaint with the state Public Employee Safety and Health Bureau yesterday, saying the city’s decision to keep schools open was violating health laws.

The hottest public building may have been the exam school, Brooklyn Technical High School, where several classrooms this week were mistakenly pumped not with air-conditioning, but with extra heat. The principal, Randy Asher, told faculty in a memo Monday that the problem was due to a valve failure that was being fixed.