Heavy rains carved a gaping sinkhole beneath the Baltimore-Washington Parkway yesterday at a U.S. government construction site, forcing authorities to close all northbound lanes at Route 32 near Laurel and delaying many travelers at the outset of the summer weekend.

The U.S. Army Corps of Engineers, which is in charge of the project, rushed yesterday afternoon to excavate loose, sandy soil underlying the roadbed and replace it with recycled concrete to stabilize the road and prevent a cave-in. The sinkhole was visible on the road median and extended underneath the highway across one lane.

U.S. Park Police officials said they were likely to reopen the northbound lanes this morning.

Thousands of motorists who ordinarily rely on the parkway to leave the Washington area were detoured starting shortly before 11 a.m. yesterday, dumping more traffic onto alternate routes. Among those affected were baseball fans en route to Camden Yards to see the Baltimore Orioles play a night game against the first-place Boston Red Sox.

On a typical day, police said, about 97,000 vehicles travel the parkway in both directions between the District and Anne Arundel County. On a Friday evening, traffic leaving the capital surges. Yesterday, alerted by electronic signs and radio traffic reports, many drivers took one of two recommended parallel routes to Baltimore -- Interstate 95 or Route 1.

Elsewhere in the region, the downpour that peaked early yesterday -- remnants of Tropical Storm Cindy -- left many neighborhoods without power and with downed trees and flooded streets.

In Old Town Alexandria, a swollen Potomac River spilled into low-lying streets and forced police to barricade several blocks near the river in a scene familiar to many residents and sandbag-ready businesses.

In Ocean City, portions of the ceiling at City Hall and at a nearby public safety building collapsed under an intense rainfall measured at five inches within four hours.

By early yesterday morning, the National Weather Service reported that 2.44 inches of rain had fallen at Reagan National Airport and 3.11 inches at Dulles International Airport. The weekend forecast called for partly cloudy skies.

In the parkway incident, an unlucky combination of rain and construction undermined the roadbed. Fred Cunningham, who manages a 29-mile stretch of the parkway for the National Park Service, said the Army Corps of Engineers was working to tunnel utility wires beneath the parkway in a project tied to nearby Fort Meade, home of the National Security Agency.

The rain, Cunningham said, washed out a large quantity of soil underneath the left edge of the road. "They must have hit some sand or soft dirt under the parkway," he said. "This is a tremendous sinkhole, definitely caused by the construction. . . . It's a disaster."

Corps spokeswoman Mary Beth Thompson said the $500,000 construction project, linking defense contractors west of the parkway to the fort on the east, was routine. She said the storm hit just when the road was most vulnerable, with most of the soil dug out at both shoulders.

"It was very bad timing," Thompson said. "A combination of where we were at the point of construction with the heavy rains, and it's just bad, bad timing."

Cunningham credited a Park Police officer, Michelle Spencer, for spotting the dangerous erosion along the median shoulder while the road itself was still intact.

Two northbound lanes and a merge lane were closed shortly before 11 a.m., police said. The southbound lanes remained open.

In the afternoon, a hole several feet deep and 15 to 20 feet wide could be seen. The Corps of Engineers labored to repair what was becoming a major traffic problem.

Crews cut into the asphalt to inspect the underlying soil, and they hauled away tons of unstable sand and dirt. The road was expected to be repaved overnight.

Cunningham and Park Police Lt. John Dewey said they expected it would reopen by about 6 a.m. today, although they cautioned that the timetable could change.

Northbound traffic was backed up for two miles at 4 p.m. yesterday as police routed cars off the parkway at Route 32. They reentered at Route 175.

The storm left about 18,000 customers in Maryland, Virginia and the District without electricity yesterday morning, with the heaviest concentration of outages in the Kensington area. Montgomery County canceled summer programs at several schools because of outages.

By late afternoon, power was restored to all but 1,000 customers in the District and surrounding counties in Maryland, Pepco reported. A spokesman for Dominion Virginia Power could not be reached yesterday evening for information on local outages.

The soggy morning also forced authorities to close several flooded Montgomery streets, and police managed traffic using hand signals at some intersections without working stoplights.

In Old Town Alexandria, police closed low-lying King, Union and Strand streets by about 8:30 a.m. because of flooding. Barricades were up on Union Street at King, Cameron and Duke streets. Several area businesses were closed as well.

Laura Mae, who lives in Old Town, ventured onto the deserted streets to take photographs and stopped at the Firehook Bakery on South Union Street.

The water was already "a half a bicycle tire deep," she said, judging from the daily bicycle commuters she'd seen struggling through the streets.

The bakery was still safe, as was Mae's condominium on the southern edge of Old Town. But she awaited high tide in midmorning.

"Our building is pretty tall, but we're starting to see the water come up over the seawall," she said. Still, Mae, like most who choose to live and work along the river, was unfazed.

"The dogs love it," she said, noting the dog walkers still out in the wind and spitting rain and pooling water.

"We're used to it. It's one of the downsides, but it could be an upside to living on the water -- you get to see nature at its best."

Staff writers Brigid Schulte and Fred Barbash contributed to this report.

Workers repair the Baltimore-Washington Parkway in the Laurel area, near Route 32, where a sinkhole forced the closing of northbound lanes. The Monocacy River in Frederick County overflows its banks after heavy rainfall Thursday and yesterday, remnants of Tropical Storm Cindy. Elsewhere in the region, the storm left many neighborhoods without power and with downed trees and flooded streets.After the northbound lanes of the Baltimore-Washington Parkway were closed, crews check the sinkhole, visible on the road median to the left and extending underneath the pavement. Officials say the sinkhole formed when heavy rains and construction undermined the roadbed.