A basement electrical fire filled a nine-story Foggy Bottom apartment building with dense smoke yesterday, forcing evacuation of scores of residents, many of them elderly, and stranding others on the upper floors without heat, light or elevators.

Acrid smoke from burning wire insulation billowed for almost three hours through the 274-unit Potomac Plaza cooperative apartment building at 2475 Virginia Ave. NW, across from the luxury Watergate hotel-apartment complex, as D.C. firefighters escorted stunned residents through darkened hallways to fresh air and used huge fans to clear the building of smoke.

One person was reported injured. A 72-year-old resident, who was in the basement when the building's central electrical panel suddenly exploded into flames, was taken to George Washington University Hospital and treated and released for smoke inhalation.

Resident manager Thomas Folk said all electrical power, except for telephones, was knocked out by the fire and that it would take at least three days to repair the gutted electrical panel in the basement and restore full power. Some limited emergency power may be available today, he said.

D.C. Fire Department investigators said they were not sure of the exact cause of the fire but said the basement electrical panel -- a kind of nerve center for the building's power -- exploded when engineers turned on the building's air conditioning motors during a routine inspection at about 10:20 a.m. yesterday.

"There was a big flash and then it exploded," said Potomac Plaza building engineer Gregory Rohrer. He said the entire 12-foot long panel, containing an intricate maze of wires and cables, was burned out by the explosion. Thick smoke forced him out of the basement, he said, and he called the fire department.

The smoke quickly spread through the building, winding up stairways and utility shafts. Firefighters with gas masks escorted residents from some of the lower floors. With the power out, elevators were not working, and firefighters using bullhorns and the building's telephone switchboard urged residents on upper floors to stay in their apartments, keep their doors closed and open their windows.

Firefighters then set up more than two dozen air blowers to suck the smoke out of the building. The process took three hours. Officials said there were no incidents of panic.

Folk said the building is occupied by about 525 people, 65 to 70 percent of them elderly. Despite the prospect of unheated and unlighted apartments, most residents said they planned to stay in the building last night. A few went to hotels or the homes of relatives.

"I'm not going to move to a hotel (and) I have no family to go to," said Laura Margulas, a ninth-floor resident.

"I'm going to stick with the ship," said Annette Bronaugh on the fifth floor. "I'll have to do without my favorite television show tonight."

"We'll read by candlelight,' said Susan Taylor on the sixth floor.