The electrical outage that darkened the Upper East Side and parts of the Bronx yesterday for nearly an hour is upping the political pressure on Con Edison and heightening fears that unless New York approves the construction of more power plants, blackouts will become increasingly frequent.

Yesterday’s outage stopped subway trains and shut down power at Gracie Mansion and at the Metropolitan Museum of Art, which was forced to evacuate more than 2,500 visitors.

The outage, following two of the hottest days so far this summer, immediately sparked outrage among some local officials who have called for the replacement of Con Edison’s chairman and chief executive, Kevin Burke, following last summer’s nine-day blackout in Queens. The Queens blackout lcame after an August 2003 blackout that spread through eight states and Canada and left the city without power for about a day.

“I’ve been expecting this,” City Council Member Eric Gioia of Queens said. “How much more do we have to take before City Hall demands a change in leadership in Con Edison?”

At a hearing the day before, City Council members had interrogated the company about a proposed rate increase for next year that could tack on about $12 a month to residential bills and $235 a month to business bills. Con Edison representatives have said they need the extra money to upgrade an aging power delivery system, which was faulted in the August 2006 blackout concentrated in Astoria and Long Island City.

“We were promised yesterday that they’ve made all these improvements,” Council Member Peter Vallone Jr. of Astoria said. “They said they would continue to provide the level of service that we’ve become accustomed to. Apparently they mean rolling black-outs and shoddy service.”

Mr. Burke said Con Edison had acted within minutes to restore power.

“I believe that we have learned some lessons from last summer, and we’re implementing those,” he said at a news conference yesterday evening. “We do everything humanly possible to provide the best service to our customers.”

Mr. Burke said the outage had been caused when breakers at an Astoria substation interrupted the transmission of electricity to stations in the Bronx and Yorkville. He said lightning in the area of the Astoria station may have contributed to the outage, but said the company had not yet confirmed the cause.

In Manhattan, the company reported, about 78,800 customers, or meters, were affected in an area bordered by Fifth Avenue, the East River, 77th Street, and 110th Street. In the Bronx, about 57,900 customers were affected in an area bordered by the Harlem River, 174th Street, 144th Street, and Park Avenue. Randall’s Island was also affected.

In the past, Mayor Bloomberg has defended Con Edison and Mr. Burke.

The chairman of the New York Affordable Reliable Energy Alliance, Jerry Kremer, predicted that the city would experience increasing numbers of blackouts this summer until the state builds more plants to serve the city’s growing power needs.

“It’s early,” he said. “Little blackouts like this are the tip of iceberg.”

A law to streamline the approval of new power plants had been awaiting approval in Albany, but it was one of the many items on which Governor Spitzer and leaders of the legislature failed to reach an agreement during the legislative session that ended last week.

On Monday, as a part of a new marketing campaign called “GreeNYC,” the mayor laid out a list of 10 things New Yorkers can do to conserve energy including unplugging phone chargers and turning off air conditioners.

Many Upper East Side shops were forced to shut down early yesterday because of the blackout, while some residents stumbled down darkened stairwells as elevators were disabled.

“It was dark and scary,” 2-year-old Nathaniel Marinaccio said after walking down 10 flights of steps with his father Lou Marinaccio, 37, from their apartment in a building on East 86th Street.

The blackout began at 3:42 p.m. and ended at 4:30 p.m., delaying trains for an hour as the afternoon rush hour began.

At the Metropolitan Museum, representatives said the evacuation was calm and orderly, although interpreters had to be deployed to help visitors who didn’t speak English.

“It was absolutely fine and lovely,” a spokesman, Harold Holzer, said.

He said had the blackout lasted longer, emergency generators were prepared to cool the air in sections of the museum containing particularly delicate works of art, such as the works on paper.

While many residents described the outage as a minor inconvenience, some braced for worse, with the hottest months of the summer yet to come.

“It’s only June. I think it’s going to happen again,” a sporting goods store employee on the Upper East Side, Satara Johnson, 25, said.