About 2,200 people were seated in the Kennedy Center Opera House last night, watching a ballet, when the huge hall was plunged into darkness as the lights went out all over Georgetown and Foggy Bottom.

The lights stayed out for more than an hour. The ballet, "Don Quixote," was canceled, along with a choral performance in the Concert Hall, which had not yet evacuated.

The screen went blank in a Wisconsin Avenue movie theater, patrons in a restaurant in the 3200 block of K Street ate an abbreviated meal, and George Washington University students crammed for final exams by candlelight.

According to the Potomac Electric Power Co. power went off at 7:59 after a fault in a 13,000 volt feeder cable set off a chain reaction that tripped out the entire Georgetown substation at 1020 33rd St. NW.

A pepco spokeswoman said the faulty feeder should have been cut out of the system by a circuit break- er, but the device did not operate. As a result, the spokeswoman said, a backup mechanism went into play, shutting down the entire substation and all of its 26 feeders, 12 carrying 13,000 volts each and 14 carrying 4,000 volts.

Power was shut off in an area roghly bounded by Reservoir Road on the north, 23rd Street on the east, the Potomac River on the south and Foxhall Road on the west.

At the Kennedy Center, a backup generator should have cut in with enough power to keep the Opera House lights burning, but the generator did not opperate. A spokesman for the National Park Service, which is responsible for the generator, said the failure could not be explained.

By all accounts, there was no light, but no panic. The orchestra stopped playing and the members of the American Ballet Theater company halted their performance and according to a witness, remained on the darkened stage.

Then ushers arrived with flash-lights, and soon firefighters arrived with emergency lights, and the dancers waltzed on stage, the orchestra began to play, stagehands took bows and the audience sang Christmas carols.

Meanwhile, in the halls, hundreds of people who had come for the 8:30 p.m. Choral Arts Society performance of Christmas music waited in darkness. Some found waiting performers and joined them in caroling.

At 8:50 p.m. Acting D.C. 6th Battalion fire chief Vincent Austin ordered the center evacuated. He said many pieces of fire equipment had been summoned (two engine companies, three truck companies and a rescue squad) and "I felt no justification" for keeping them out of service."

"We didn't know whether the lights would go on in 10 minutes or 10 hours," he said.

A Kennedy Center spokesman said that ballet refunds could be obtained at the point of ticket sale and that an attempt would be made to rescheduled the choral performance.

At Chadwick's restaurant, 3205 K St. NW, meals could not be served but patrons received free drinks. The 46 people who had been watching "Emmanuelle II" at the Georgetwon Theater got refunds.

At 9:25 p.m. about 50 carolers at the Kennedy Center were trilling "'tis the season to be jolly . . .," as the lights went on again.