A series of explosions and a seething electrical fire broke out beneath Columbia Road NW yesterday, blocking traffic, sending billowing black smoke into the street for more than an hour and causing power to be cut off to hotels, homes and businesses for blocks around.

Within minutes of the outbreak of the blaze, nearby residents and merchants and their customers were urged by police to leave their stores and homes to escape swelling clouds of thick smoke.

Soon afterward, as power was shut off over a wider area, hundreds of hotel guests dined by candlelight, an uncompleted video game faded from the screen before the eyes of an anguished 12-year-old boy, and fire department rescue trucks rolled through streets without traffic signals to free people from elevators stalled in blacked-out buildings.

The explosions and fire, which occurred in Potomac Electric Power Co. lines in the 1700 block of Columbia Road NW, came two days after a Pepco transformer exploded in Southwest, cutting power to thousands of stores and homes and briefly trapping President Reagan in an elevator.

No specific causes have been reported for either incident, but authorities said there appeared to be no connection between them.

Yesterday's incident began shortly after 4 p.m. on the north side of Columbia Road near 18th Street NW, a principal intersection in the bustling Adams-Morgan area, when witnesses reported hearing a crackling noise followed by what several described as one or more powerful blasts.

"There was a boom," said John Orcino, of Avignone Freres, a caterer and restaurant at 1777 Columbia Rd. "I saw seven or eight manhole covers go eight feet in the air."

The explosion, according to Frank Brailer, assistant manager of the nearby Giant Food store, "sounded like a cannon going off," and soon, according to one of the 3rd District police officers sent to the scene, "the smoke was just pouring out" of the manholes, along with tongues of flame that leaped six to eight feet.

The blaze, according to a Pepco spokeswoman, began in two underground cables, one at 120 volts and the other at 208 volts.

About two hours after the blaze began, the spokeswoman said, Pepco cut off power in the surrounding area to permit the cables involved to be isolated, and by about 6:30 p.m. the fire was out.

"It pretty much burned itself out when the power was cut off," said Capt. Joe Kerr, the acting 4th Battalion chief who commanded three dozen firefighters and one dozen pieces of equipment sent to the scene largely as a precaution.

Meanwhile, rescue personnel, hoteliers, area residents and businessmen were coping with ice, candles and a variety of other backup equipment as Pepco cut off power to 2,358 premises in and near Adams-Morgan at 5:55 p.m. and then cut off about 600 more in Mount Pleasant a few minutes later.

At the Shoreham Hotel, rescue workers released seven persons from two elevators by using special keys that opened the shaft doors at the ground floor, according to Lt. Robert Del Grosso. Passengers also were freed from elevators in apartment buildings in the 1900 and 2000 blocks of Connecticut Avenue, he said.

At the Sheraton-Washington Hotel, 1,300 persons were dining in a ballroom when, according to assistant manager Judy Havill, the lights went out.

"We turned it instantly into a candlelight dinner," she said.

At the Giant store, employes said 3,000 pounds of dry ice were brought in to keep meat and produce fresh. At Avignone Freres, Orcino said he had sent for a mobile electric generator from Pennsylvania, and was planning to bring in a freezer truck for temporary storage of 22,000 gallons of ice cream.

Almost all power was restored by 10:36 p.m., Pepco said. Some places on Columbia Road remained dark, however, as repairs continued.

Washington Gas Light Co. employes checked last night for signs of possible fire damage to buried gas line.