When the electric utilities that serve the District, Baltimore and their Maryland suburbs shut off power to more than 100,000 customers yesterday, it marked the first time since 1969 that blackouts were imposed in this area because of a shortage of generating capacity, utility officials said.

Although the service interruption was brief, it showed dramatically that a combination of high demand and equipment failure could push the region to the brink of widespread electrical failure.

Service to most of the affected homes and businesses was restored within an hour. A break in the weather and emergency repairs to power plants that failed yesterday should prevent a repetition of the problem today, utility officials said. But the incident illustrated the thinness of the reserve capacity at Potomac Electric Power Co. and Baltimore Gas and Electric Co.

Those utilities, which serve the District, its Maryland suburbs, Baltimore and most of central Maryland, and Virginia Power, which serves Northern Virginia, reported electricty demand near an all-time high as temperatures near 100 degrees prompted an increase in the use of air conditioners.

Some Virginia Power customers lost power because of last night's storm, but the utility, which has added extensively to its generating capacity since last summer, did not black out any customers intentionally.

Throughout the early afternoon yesterday, the utilities took steps designed to restrain demand. They reduced voltage by 5 percent to hundreds of thousands of customers. They asked radio and television stations to broadcast appeals to customers to turn off lights and air conditioners and postpone doing laundry. They appealed to large commercial and governmental consumers for voluntary reductions in electricity usage. They cut off power to central air conditioners and electric water heaters.

But while they were doing that, generators broke down at Pepco's Chalk Point plant in Prince George's County and at Baltimore Gas & Electric's Wagner plant in Anne Arundel County. At 3:30 p.m., Robert Woodward, general manager of the PJM Interconnector, a multi-state power pool based in Valley Forge, Pa., ordered Pepco and BG&E to begin rotating blackouts to prevent failure of the regional grid. Pepco shut off 70,000 customers, BG&E 35,000, the utilities said. Virginia Power, which is not part of the multi-state pool, did not cut off any customers.

"We wanted to stay in control" and not let the situation get out of hand, Woodward said. "You're dipping down into the emergency procedures pretty far."

The demand surge hit Pepco and BG&E when they were vulnerable, with major power plants out of service. A Chalk Point unit that generates 614 megawatts of power, more than 10 percent of Pepco's total capacity, was shut down because of a tube leak, Pepco spokeswoman Nancy Moses said. Both units of BG&E's nuclear plant at Calvert Cliffs, with a combined capacity of 1650 megawatts, have been closed for more than a year. Calvert Cliffs represents about one-third of the utility's total capacity, a company spokesman said.

In midafternoon, another Chalk Point unit "tripped off because of a water pump problem," Moses said. Shortly afterward, a 135-megawatt BG&E unit at Wagner went out of service. A company spokesman said he did not know the cause of the failure or how long it would be down.

Power was restored to the customers who had service interrupted after one of the Chalk Point units was repaired, Woodward said.

Moses said Pepco's emergency shutoff plans require that power not be interrupted to any regional substations that serve hospitals, Metro trains or high-rise buildings. As a result, she said, the neighborhoods cut off are generally residential communities that do not include any such facilities.

Pepco's maximum generating capaity of 5,909 megawatts includes an operating capacity reserve of about 10 percent, spokeswoman Dana Grabiner said. That is well below the recommended industry standard. With demand rising steadily because of residential and commercial growth in the Washington suburbs, the utility is planning to construct four gas-turbine generators at Chalk Point to add 320 to 400 megawatts of capacity, and to gain another 500 megawatts through incentives to reduce consumption, she said.