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By Daniel Trotta

NEW YORK, June 27 (Reuters) - A power outage struck Manhattan's wealthy Upper East Side during a heat wave on Wednesday, snarling subway service during rush hour in another embarrassment for utility Con Edison (ED.N: Quote, Profile, Research).

The outage left half a million New Yorkers without power for 50 minutes. Traffic signals died, classrooms went dark, the fire department rescued people from elevators and the Metropolitan Museum of Art was evacuated in scenes that raised fears of another major summer blackout.

Grumpy New Yorkers recalled epic blackouts of the past on the second humid day in a row of temperatures topping 90 degrees F (32 C).

Consolidated Edison Inc. said some 136,700 customers were without electricity in the Upper East Side and in the Bronx, including the neighborhood around Yankee Stadium, after a transmission disturbance. It was investigating the cause.

Typically, each customer accounts for a home of at least three people, meaning some half a million New Yorkers were without power. Con Edison provides power service to 3.2 million customers in New York City and surrounding areas.

The incident is likely to raise more tough questions from New York officials of the quality of Con Edison's service.

Last year, parts of the New York borough of Queens went without electricity for more than a week, leading to public hearings and political pressure on the firm, which is regulated because it is a natural monopoly in its areas of service.