An underground explosion and fire yesterday afternoon near Pennsylvania Avenue and 20th Street NW sent 50-pound manhole covers flying 10 feet into the air and forced closure of streets and the evacuation of a two-square-block area around the site.

The fire, which was touched off by the blast about 5:10 p.m., continued to smolder for more than two hours as it radiated outward from the intersection up and down 20th Street and Pennsylvania Avenue, sending black clouds of smoke billowing from manholes along Pennsylvania Avenue.

Utility officials on the scene last night were still trying to determine the cause of the initial blast and a "fireball" that witnesses said shot 100 feet into the air from a manhole.

Fire officials said the explosion apparently occurred in electrical lines or apparatus and traveled through the lines about a half-block in all four directions.

They said the fire was apparently feeding on rubber tubing and other electrical insulating materials beneath the streets.

All gas and electrical lines in the area were shut off, officials said, and the fire died out about 7:30 p.m. No injuries were reported

Firefighters said witnesses told them the blast was accompanied by a "fireball" two feet across that blew off a manhole cover at 20th and Pennsylvania.

The witnesses said the fireball rose about 100 feet into the air and was followed by thick clouds of black, acrid smoke. Two other manhole covers at the intersection and another a half block away were shot about 10 feet into the air by the force of the blast, fire officials and witnesses said.

"At first we thought it was some kind of a bomb blowing up the building," said Dominique D'Ermo, owner of the fashionable Dominique Restaurant near the intersection.

Police and fire officials allowed D'Ermo to continue serving the 20 or so customers in the restaurant when the explosion occurred, but they would not allow anyone else to enter, he said.

Police rerouted traffic in a two-square-block area along Pennsylvania Avenue, from 21st Street east to 19th Street between H and I streets, while firefighters went from building to building in the area and urged occupants to leave for their own safety.

Firefighters who entered a nearby manhole said they were able to see what appeared to be a break in a high-voltage line beneath 20th and Pennnsylvania, and that the blast may have been sparked by electrical arcing. Loud "pops" at five-to-10-minute intervals continued for about two hours.

Nancy Moses, spokeswoman for the Potomac Electric Power Co., said last night that the fire may have started in low-voltage lines and spread to some adjacent high-voltage feeder cables, but that the explosion source was still unknown. David W. Masters, vice president for electrical systems for Pepco, whose headquarters is about a block from the explosion, said last night that four 13-kilovolt power lines were "irreparably" damaged.

He said the lines feed power to about eight blocks of office buildings, restaurants and other commercial establishments, and that Pepco crews would attempt to replace the lines overnight.

"I don't think we'll ever be able to determine what caused the explosion because the damage was so extensive," he said.