Swift and fierce thunderstorms pounded the Washington area last night, leaving more than 150,000 homes without electricity and injuring four people in Loudoun County who were struck by lightning.

Utilities officials said that a relatively small number of customers could be in the dark until tomorrow night. By early today, crews had restored electricity to tens of thousands of households, and about 115,000 remained without power.

"We're saying 8 p.m. Friday for [restoration of power to] the very last customer,'' said Pepco spokeswoman Mary-Beth Hutchinson. "Most people, I feel certain, will be back well before that. We're hoping for better news for most people tomorrow."

In Northern Virginia, where about 20,000 customers were without power early today, all outages are expected to be repaired by tonight, a Dominion Virginia Power spokesman said.

In Loudoun, one person suffered cardiac arrest when struck by lightning in the Lucketts area and was revived by rescue workers, said Mary Maguire, a spokeswoman for the fire and rescue department. She declined to confirm a report that the four were teenagers who had gathered near a tree.

The four were transported to hospitals, and their conditions were not available. The incident occurred about 5:40 p.m. in the 14600 block of New Valley Church Road, about five miles north of Leesburg. A dog that was with them died after being struck by lightning.

The storms were triggered by a cold front entering the region from the west that clashed with the hot, humid air mass that had stalled over the region in recent days, said Calvin Meadows, a meteorological technician for the National Weather Service in Sterling.

Temperatures plummeted about 20 degrees in three hours, bringing readings in the low 70s for the first time in several days.

Early today, most of the remaining outages were in Maryland. More than 36,000 Pepco customers were without electricity in Montgomery County, and nearly 43,000 Baltimore Gas and Electric Co. customers lacked power in Anne Arundel, according to the utilities' Web sites. More than 22,000 outages were reported in Prince George's, along with about 2,200 in the District.

The severe heat and the added demand for electricity in recent days might have swollen power cables, making them more susceptible to water damage from the storms, Hutchinson said.

Before the searing heat abated, about 300 people at the Boy Scout National Jamboree at Fort A.P. Hill in Caroline County were treated for heat-related conditions, Scout officials said late last night. Most were treated at the on-post medical clinic. About 240 had been released, officials said in a statement on a Web site, with the "few" who were admitted to hospitals reported to be in stable condition.

The clashing weather systems triggered spectacular lightning displays, and also ignited house fires and blasted power boxes. Utility crews scrambled to identify short-circuited wires and blown fuses.

The storms snapped trees and produced strong, gusting winds and hail in many areas. By late last night, the fierce weather had largely moved out of the region.

Along the outer loop of the Capital Beltway, drivers heading home witnessed vivid vistas of forked lightning as they dodged tree branches that blew before slow-moving traffic. Downed tree limbs and other debris blocked roadways in the District's Rock Creek Park, along Maryland's Route 29 corridor and the major arteries of Prince George's.

Brian Mooney, 29, of Germantown said that he usually loves thunderstorms and that he had looked forward to last night's boomers.

"This was supposed to be our relief from the heat," Mooney said by telephone, before lightning burned a hole in the roof of his neighbors' condominium building.

Mooney said the lightning crash next door knocked him off his feet. The resulting fire in the 12900 block of Church Hill Ridge Circle left several families without housing and caused about $200,000 worth of damage.

"It was heavy-duty. There's no joke -- the lightning put a car-sized hole in the roof of that building," Mooney said.

The weather front was a part of long band of showers and thunderstorms that stretched from Pennsylvania to eastern Tennessee, reportedly causing similar wind damage in those areas, the Weather Service said.

The storms produced golf-ball-size hail in areas of Montgomery and dumped about a half-inch of precipitation in most areas, as gusts of up to 54 mph kept the storm clouds from lingering.

"The storms were moving at a rather brisk pace," the Weather Service's Meadows said.

Meadows said the storms are expected to bring daily high temperatures into the mid-80s for several days, bringing "relief from the hot, muggy conditions."

Prince George's fire officials reported two electrical fires caused by lightning or power surges. They said the storm hit the county hardest in the Hyattsville and Landover Hills areas.

No injuries were reported in any of the fires, officials said.

Staff writers Nick Anderson and Hamil R. Harris contributed to this report.

Andrea Schutz, 23, waits at a bus stop in Dupont Circle. As the rain stopped, she decided to walk home.