NEW YORK (CNN) -- Scattered power outages in New York inconvenienced tens of thousands of travelers Tuesday as Con Edison struggled to meet high demand for power in the face of sweltering temperatures.
As temperatures threatened to break 100 degrees, Mayor Michael Bloomberg said the power grid is holding up, but he appealed to New Yorkers to conserve energy. "Keep your thermostat at 78 degrees or higher," he said. "Also, see if you can wait to run appliances."
The mayor announced the city was opening 300 cooling centers -- air-conditioned locales open to the public -- and would keep them open until 9 p.m.
At LaGuardia Airport in Queens, hundreds of passengers became stranded at 8 a.m. Tuesday, when power outages interrupted security screening at the Marine Terminal and darkened part of the main terminal.
Delta Air Lines canceled flights in its shuttle operation to Washington and Boston, Massachusetts, which is run out of the Marine Terminal on the airport's west side, said spokesman Anthony Black.
Outbound passengers could not be screened.
Inbound flights were able to land and passengers could deplane, but their luggage had to be taken to the baggage claim area by hand, since the conveyor belt was not moving, he said.
At the main terminal, hundreds of American Airlines passengers were stranded by power outages. The airline canceled all of its flights scheduled to depart from the airport until 5 p.m., and said its representatives were meeting with Con Edison representatives to determine the cause of the problem.
A spokesman for Port Authority of New York & New Jersey, which runs the region's three metropolitan airports, said the power failure in the main terminal occurred at 8 a.m., and followed another power failure from 8:30 p.m. Monday until 12:30 a.m. today.
Passengers were being booked on other carriers, spokesman Mark Lavorgna said.
The darkness led to fumbling inside the restrooms. "How do you use the facilities here?" one stranded passenger asked. "There's no light."
Battery-powered emergency lights provided dim illumination throughout American's section of the terminal.
The contrast between it and the rest of the terminal, which has power, was stark: A bar located steps from the darkened concourse was packed with customers, its 31 televisions turned on.
American Airlines said it was diverting some incoming flights to nearby John F. Kennedy International Airport and Newark-Liberty International Airport.
Subway riders also were inconvenienced. New York Transit Authority spokesman Charles Seaton said that from about 9:30 a.m. until shortly before 11 a.m., service along the 1, 2, 3 and 5 subway lines was suspended.
Seaton said he had "no idea" what caused the disruption, which affected the signals that control the trains and not the third rail, which powers the trains.
Afonso Quiroz, a spokesman for Con Edison, said the scattered outages extended to surrounding neighborhoods in Queens, including Astoria, Long Island City, Sunnyside, Woodside and Corona. He urged customers to refrain from using electrical appliances.
By early afternoon, it was 95 degrees in Manhattan, with a predicted high of 102 by midafternoon.