Power was knocked out to tens of thousands of homes, a house caught fire and part of a warehouse broke up last night as severe thunderstorms rolled relentlessly across the Washington region.

But it appeared that the area was spared greater damage. Weather forecasters warned several times during the evening that tornadoes were developing, but in the end, there were no reports that any touched down.

With spectacular flashes of lightning playing in huge thunderheads, the storms brought hail, high winds and bursts of fierce rain to most of the region, as they swept from West Virginia's panhandle across Northern Virginia and into Maryland.

The total number of homes and businesses without electricity in Washington and the close-in suburbs remained at fewer than 10,000 at any one time.

At several points, Dominion Virginia Power said that about 6,000 of its customers were in the dark; Pepco reported about 2,000 outages late last night.

However, in the area of Maryland served by Baltimore Gas and Electric Co., 51,000 customers had lost power in Baltimore and Baltimore County alone.

In the Potomac area of Montgomery County, the roof of a 21/2-story house caught fire shortly after 8:30 p.m. when it was struck by lightning as a storm passed overhead, a county fire and rescue spokesman said.

The house, in the 9500 block of Hemswell Place, suffered $1.25 million in damage, said fire spokesman Pete Piringer. Three people were believed to be in the house when the fire broke out, but no injuries were reported.

In the District, firefighters said a lightning strike on the upper portion of a five-story building sent debris cascading onto the street and sidewalk in the 2100 block of Georgia Avenue NW.

Police said that the avenue would be closed between V and W streets NW until the damage to the building could be assessed. The brick-and-concrete building, in the vicinity of Howard University, was believed to be a warehouse.

Of the region's three airports, delays of flights to and from Baltimore-Washington International were the longest, at more than 45 minutes, according to the Federal Aviation Administration.

The region was pelted with hail, measured at nine-tenths of an inch in Clinton, an inch at Route 50 and Prosperity Avenue near Merrifield and 11/4 inches at Interstate 395 and Seminary Road in Alexandria.

Throughout the evening and into the night, radar indications that the storms were beginning to rotate led to the issuance of tornado warnings. Alerts were broadcast at various times for Loudoun, Fairfax, Frederick and Prince George's counties and for Alexandria. Sightings of funnel clouds were reported in the Springfield, Langley, Lorton and Annandale areas of Fairfax.