Updated

A power outage darkened a large swath of Manhattan and the Bronx on Wednesday, knocking out traffic lights, cutting subway service and forcing the evacuation of the Metropolitan Museum of Art on one of the hottest days of the year.

"The entire neighborhood is out," said Phil Quintana, a building superintendent on 94th Street on the Upper East Side.

Con Edison said the blackout affected 136,700 customers in all, or more than 500,000 people.

The cause was under investigation, but spokesman Chris Olert said it was some sort of transmission disturbance. He didn't know whether the heat was a factor. "We won't even speculate on the cause yet," Olert said.

The city was in the second day of temperatures well over 90 degrees.

The outage stretched from the east side of Manhattan to the Bronx. The office of the Bronx borough president said multiple spots were affected, including the area around Yankee Stadium.

The Metropolitan Museum of Art was evacuated around 4 p.m. and visitors were forced to sit on the outside steps in the sweltering heat. The street lights up and down Fifth Avenue were dark.

The power outage caused suspensions and delays along the city's subways. New York City Transit spokeswoman Deirdre Parker said the trouble started at 3:40 p.m.

At Gristede's Pharmacy at Lexington and 89th Street, power was out for a half hour until about 4:20. They operated on emergency generator.

"People came in off the street and we were selling flashlights, bottled water, candles, ice," said Barry Newman, the supervising pharmacist.

He said the traffic lights were off on the street, "and people stood outside their apartment buildings, looking nervous. Everyone was saying, 'What's going on? What's going on?"'

The Metro-North commuter railroad, which serves the northern suburbs, had to reduce the number of trains it was using, resulting in delays and crowded trains as the evening rush-hour approached, said spokeswoman Marjorie Anders.

The power outage was reminiscent of previous summer blackouts that struck New York City.

Last summer, about 174,000 people were affected by a blackout in Queens. At the height of the crisis, 10 of the 22 feeder cables that carry electricity through the Con Ed system were malfunctioning. Residents sweltered without air conditioners on some of the hottest days of the years, and estimated business losses ran into the tens of millions of dollars as stores were forced to throw out perished goods.

The Public Service Commission issued a blistering report this year, charging the local utility's performance was "unacceptable and a gross disservice to its customers."

The report detailed a litany of mistakes that Consolidated Edison made and said the company needed to make "critical and substantial" improvements.

New York was also hard hit by a 2003 blackout that cut power to a large chunk of the Northeast.

A customer can represent anything from a single-family home to an entire apartment building, roughly translated to four people per customer.