SEATTLE (AP) _ An underground electrical fire caused a 40-block power outage downtown, sending many workers home early, and utility officials said they couldn't start working on repairs until this morning.

Power was not expected to be restored to some areas until tonight, according to Seattle City Light spokesman Hugh McIntosh.

City Light crews scrambled today to locate and deploy generators for some hotels and buildings that house mostly poor, elderly people, he said.

About 95 percent of the major buildings in the area have at least some auxiliary power supply, officials said.

The affected area included the downtown retail, financial and hotel center, along with some residences. Long-distance phone service also was disrupted for some customers.

The number of affected residents, businesses and hotels was undetermined, since many establishments continued operating using backup generators.

Fourteen officers were added to the usual 10 on patrol in the blacked-out area overnight, and no looting was reported, Officer Steven Thomas said early today.

Fifty-five additional police officers patrolled and directed rush-hour traffic on Wednesday, Fire Marshal B.L. Hansen said.

City Light officials said it was the worst outage the municipally owned utility had experienced, eclipsing an 80-block downtown blackout that lasted five hours in 1966.

A total of 3,000 feet of cable had to be replaced and work couldn't begin on repairs until this morning, according to spokeswoman Dorothy Nelsen.

''This could cost me $5,000 to $10,000,'' said Mike McAlpin, owner of Mama's Mexican Kitchen.

Others fared better. David Sivinski, proprietor of a propane-powered popcorn wagon, said his booming business represented ''the revenge of low- tech.''

Fears of a toxic chemical release proved unfounded when it was determined that no transformers ruptured during the fire, easing concern that oil containing potentially dangerous PCBs had burned, McIntosh said.

Firefighters originally thought transformer oil was burning when they saw black smoke, said Fire Department spokesman Roy Davis. They learned it was the plastic insulation around the transformers that ignited, McIntosh said.

The fire was confined to the underground vault. Firefighters took two hours to extinguish it by filling the chamber with carbon dioxide. No injuries were reported.

The loss of power and long-distance phone service prompted some businesses to send employees home early. But mass evacuations were avoided because so many major buildings in the area switched to their backup systems.

Two window washers, stuck on an electrically powered scaffold outside the 14th floor of a building, lowered themselves by hand on a bosun's chair, drawing applause from spectators.

The Claremont Hotel was evacuated because of heavy smoke emanating from manholes in the street. The Bon Marche department store also was evacuated because a backup power system failed, said a woman who answered the telephone there and identified herself as an employee.

The Warwick Hotel distributed 150 flashlights to its guests, said Cyndy Sparhawk, the hotel's sales director.

Auxiliary power meant Warwick guests had elevator service, hot and cold water and hall lights Wednesday night, but room lights will be out until the power service is restored, said Kevin Klein, a front office assistant manager.

Jerry Wolsborn, managing director of the 865-room Westin Hotel, said emergency lighting and one elevator in each of the hotel's two high-rise towers were sustained with backup systems.

U S West Communications, the regional phone company, restored its long- distance phone service by Wednesday evening, and was trying to provide emergency generator power for other long-distance carriers, said spokesman Vic Kucera.