Fierce winds swept through the Washington area yesterday afternoon, ripping off roofs, shearing tree branches and leaving thousands without electricity. On Virginia interstates, two large green directional signs broke free, sailing into the traffic below but, remarkably, injuring no one.

A 19-year-old construction worker in Alexandria wasn't so fortunate: A strong gust toppled a 30-foot-high block wall on the man, who had to be driven to the hospital because medevac helicopters were grounded. The worker, whose name wasn't released, sustained facial and back injuries, which officials said did not appear life-threatening.

On Interstate 81 near Harrisonburg, Va., meanwhile, two tractor-trailers were blown over by the heavy winds; again, no one was seriously hurt.

Closer to home, two overhead signs--one on the Capital Beltway, the other on Interstate 395--were blown onto the busy highways, narrowly missing vehicles. In Arlington County, North 15th Street near Court House Road was closed after plywood blew off a high-rise under construction, and Prince George's County public schools canceled all evening activities because of the unsettled weather.

"It's incredible," said Virginia State Police spokeswoman Lucy Caldwell. "The winds have been extremely strong, almost resembling a tornado."

Forecasters blamed the blustery conditions on a cold front that crossed the Blue Ridge early yesterday afternoon with its sights set on Washington. "It's windy everywhere," said Dewey Walston, a forecaster with the National Weather Service. "There are gusts up to 50 or 55 miles per hour."

Power companies reported thousands of outages. Potomac Electric Power Co. said 17,800 customers--more than 10,000 of them in Prince George's--had no electricity as of mid-afternoon. By nightfall, 6,000 customers were back on line.

Virginia Power reported 7,500 outages, about half of them in Fairfax County and Fairfax City. Farther west, in Manassas, 3,000 Northern Virginia Electric Cooperative customers lost service.

Virginia Power's Le-Ha Anderson said crews were working into the night to repair downed and damaged lines, but that continuing gusts were hampering the effort. As of 6:30 p.m., nearly 4,000 customers still lacked power. "Until the winds die down, it's hard to say when it will be restored," Anderson said.

Police departments around the area stationed officers at dozens of intersections where traffic lights were dark, and highway crews were kept busy moving tree branches off commuter routes.

Caldwell said the winds tore off a large sign suspended over Shirley Highway (I-395) near the Pentagon and another on the Beltway near Gallows Road. Incredibly, she said, no motorists were injured. "They just blew down and people were able to dodge them," Caldwell said.

Prince George's firefighters reported receiving more than 100 wind-related calls between 1:30 and 3 p.m. "We were hit pretty heavy and hard throughout the afternoon," said Capt. Mark Brady. "It's like we experienced a real intense thunderstorm, just without the rain and lightning."

A 42-year-old woman was taken to the hospital after being hit in the side by a green awning that was blown off the entrance of Jasper's Restaurant in Greenbelt, Brady said. She was listed in good condition last night at Doctors Community Hospital in Lanham.

Forecaster Walston said the area will be spared more gusty blasts today but that the accompanying cold will linger. Temperatures are expected to rise only into the mid-30s today, he said.

Staff writers Allan Lengel, Angela Paik, Manuel Perez-Rivas, Jamie Stockwell, Linda Wheeler and Graeme Zielinski contributed to this report.

CAPTION: Trees felled by strong winds cover the back yard of a home on Falling Creek Court in Annandale. "The winds have been extremely strong, almost resembling a tornado," said a state police spokeswoman.