The District's 911 emergency telephone lines and most city police radio channels were out of service for nearly two hours last night after an electrical malfunction shut down power to the sixth-floor communications center at police headquarters, D.C. police officials reported.

The shutdown caused difficulties for people seeking police help and in the dispatching of police, fire and ambulance units, but officials said that there seemed to have been no major crimes or emergencies during the period and no serious disruption of services.

The power failure, which was centered in the computer-aided police and fire communications system on the top floor of the City Municipal Center at 300 Indiana Ave. NW, occurred about 8:45 p.m., according to Lt. Hiram Brewton, a police spokesman.

Brewton said police and fire telephone and radio operators worked through most of the shutdown using an emergency backup generating system that kept only two of 14 radio channels normally operated from the building on the air.

The shutdown, still not fully explained early this morning, also knocked out police information computers that provide officers with instantaneous data on warrants, drivers' permits and auto registration, as well as patrol car locations.

During one period, Brewton said, the entire sixth floor was totally without electricity and dozens of officers manning telephones and radio channels were forced to sit in the dark. The backup generator came on after about five minutes, but apparently because of a separate malfunction in it, there was only sufficient power for lighting and for two of the radio channels, Brewton said. A second backup system, consisting of two linked 12-volt auto batteries never came into play, officials said.

As Pepco workers and building engineers tried to locate the source of the electrical failure last night, police officials sought to alert the public to the problem through local television and radio stations, and persons requiring emergency assistance were told to telephone police stations in their local districts. At the same time, the fire department, whose truck and ambulance dispatchers are housed in a separate building, set up two special temporary phone lines to handle emergency calls.

"Everything worked fine. We did not have any problems," said Battalion Fire Chief Michael Tippett. "We had about 30 calls during the outage and everything went smoothly."

Power was fully restored to the building at about 10:30 p.m., according to a police spokesman, who added: "We still don't know what caused it."

Pepco spokeswoman Carol McCall said early this morning that the problem originated with police-owned equipment.

"We could tell that power was being supplied to the facility and determined that it was not a problem on Pepco equipment," McCall said. However, Pepco crews remained at police headquarters to assist city employes in correcting the power problem, she said.