A major electrical problem in a sewer caused an explosion at the corner of Mt. Auburn and Holyoke Streets yesterday afternoon, blowing a manhole cover off and sending gaseous fumes into nearby buildings. The explosion forced the evacuation of the Harvard Band room and a nearby restaurant and raised fears of a Square-wide power blackout.

At approximately 1:45 p.m., the Cambridge Fire and Police Departments responded to calls reporting the explosion and immediately roped off the area surrounding the manhole along with Holyoke Street, leading to the Hasty Pudding building.

Lisa M. Card '01 said she and 20 other students were in the band room, in the basement of 74 Mt. Auburn St., when the power went off and the fire alarm sounded. Card said she left the building to see black smoke pouring from the manhole just outside.

"The [manhole] cover flew off, and we heard it land. Lots of thick smoke was coming out," said Daniel J. Rhodes '01.

The explosion sent gaseous but ultimately harmless fumes into surrounding buildings. Firefighters investigated individual complaints in Claverly Hall, Apley Court and the Spice restaurant, Captain Pat DeMaio said.

Gas levels were high enough in Spice that the fire department evacuated the restaurant for roughly 45 minutes, herding employees into the nearby Fleet Bank ATM to keep them warm in yesterday's frigid weather.

The odors in surrounding buildings did not pose a threat to occupants, officials said, but were simply a by-product of the initial explosion.

Glenn Wicks, yesterday's on-call official for the electrical company NSTAR, said a major electrical cable was "burned up" as a result of an abnormal surge of power through the wire and was the cause of the explosion that blew off the manhole cover.

Mike Nicoloro, the wiring inspector for the city of Cambridge, said the explosion could have been caused by a variety of causes, from an aging electrical cable to an abnormally high demand for electricity from the surrounding buildings.

"It's cold today, and all the heating units are running," Nicoloro said.

By 2:30 p.m., the fire trucks were gone, but work was just beginning to fix the electrical problem.

Crews from NSTAR worked well into the night, re-routing power so that workers could repair the burned cable.

Wicks was concerned that repairs might prove so difficult that all of Harvard Square would lose power.

"This whole area could be knocked out," he said. "But we can't tell until we get down there."

But as of 7:30 p.m. yesterday, NSTAR workers said power had been successfully re-routed, and they did not expect any significant interruptions in service as they repaired the damaged cable.

--Staff writer Joyce K. McIntyre can be reached at jmcintyr@fas.harvard.edu.