Thousands of customers in the Washington area remained without power late last night -- testing the patience of utility customers as crews raced to repair power lines downed by a string of powerful thunderstorms that moved through the region Wednesday evening.

Utilities officials said crews made significant progress yesterday, aided by cooler weather after several days of sweltering heat. About 6,600 customers remained without power in Montgomery County, 9,600 in Anne Arundel County, 5,600 in Prince George's County and about 600 in the District. In Nothern Virginia, where the outages were fewer, power had been restored to all but 191 customers by 10 p.m.

Outages from violent storms this week provided the first major test for Pepco and other Maryland utilities since the state Public Service Commission took them to task for their response in 2003 to Hurricane Isabel, which left more than a million Maryland residents without power -- some for more than a week.

Bob Dobkin, a spokesman for Pepco, said the utility dispatched 145 crews to deal with 60,000 outages and more than 400 reports of downed lines in an effort to restore power to all customers by 8 p.m. today.

"The power goes out whenever there's a good-size storm," said Bruce Shulman of Wheaton, who is among the homeowners who said they are frustrated by the frequency of the outages.

Phyllis Fingerhut, 69, and her husband had a house full of guests from out of town when the power went off about 1:30 a.m. Saturday. The power was still off the next day as the temperature outside soared. The Fingerhuts, who are Orthodox Jews, could not drive on the Sabbath to somewhere cooler or call Pepco to complain. "It was horribly uncomfortable," Fingerhut said.

The power came on about 2:30 p.m. Saturday but went off again about 6:30 p.m. Wednesday as another storm swept in.

"I lived in Chicago most of my life and New York a few years and never have experienced this," Fingerhut said. "Chicago is a city 10 times this size and in all the years I lived there, the power went out maybe two, three times."

As many as 150,000 homes in the region lost power Wednesday. Fewer than 20,000 Baltimore Gas and Electric Co. customers across Maryland were without power last night, and the utility said that all customers should have electricity by this afternoon.

Northern Virginia had fewer problems, despite lightning strikes in Loudoun County that left one man hospitalized in critical condition yesterday. Dominion Virginia Power said about 20,000 customers lost power, mainly in the Leesburg area.

After Isabel, the worst storm to hit the region in 50 years, Pepco customers accused the utility of repairing storm damage too slowly and failing to provide prompt, reliable estimates of when lights would come on.

The Maryland Public Service Commission, which regulates energy providers, ordered Pepco and other utilities to improve their response to storms. Commissioners said the companies should improve communications with customers, manage tree growth and look for opportunities to bury power lines.

The utility has taken a number of steps in response, Dobkin said, including improving its computerized system that takes calls from customers and analyzes the information. The system is designed to handle 100,000 calls an hour. Even so, some customers are frustrated.

Shulman called the number at 1:45 a.m. Saturday, just after a swift and violent storm knocked out power to his Wheaton neighborhood. The computerized voice told him the power was estimated to be back up at 7 a.m. It didn't come back until 12:30 p.m.

His power was out for nine hours after Wednesday's storm. This time, the system wouldn't let him through. "You simply got a message saying, 'Due to the high volume of calls, we are unable to direct your call,' '' he said.

Dobkin said the utility is continuing to make improvements and is working with local governments and homeowners to discuss the possibility of burying power lines.

"That's not a decision Pepco can make by itself," he said. "It has to be done with the community."

Repair time estimates have also improved, Dobkin said. After Saturday's storm knocked out power for about 55,000 customers, Pepco estimated that it would have power fully restored by 6:30 a.m. Monday. It missed by about 1,000 customers, Dobkin said.

Allen Staggers, spokesman for Allegheny Power, reported that the storm knocked out power to 2,900 of its Maryland customers, mostly near Frederick County. Last night, about 36 customers were still in the dark.

Linda Foy, a spokeswoman for BGE, about 400 people were dispatched to repair outages in Anne Arundel and parts of Prince George's. "A lot of people are working 12-hour shifts or even longer," she said.

Power should be restored to most customers by early today, Foy said.

Foy said some of the changes implemented after Isabel improved the utility's response to Wednesday's storm. She cited a mobile dispatch system installed last summer that allows trucks in the field and workers in the central storm center to communicate instantaneously via computers.

About 3,500 Southern Maryland Electric Cooperative customers in Prince George's, Charles and St. Mary's counties lost power Wednesday night, said spokeswoman Terry Ressler. She said power was restored by 6 a.m. yesterday.

Utilities were under stress before the storms because of the tremendous energy demand during the heat wave. BGE, Southern Maryland Electric and Dominion Virginia Power reported record-high demands Wednesday and reduced their voltage.

The voltage reduction ended about 5:30 p.m., when clouds rolled in and the temperature began to drop.

"But, of course, that came at a price," Foy said. "Instead of heat, we had to deal with thunderstorms."

Staff writers Rosalind S. Helderman and Nelson Hernandez contributed to this report.

Greg Mitas of Kuhn's Tree Service in Silver Spring crawls through a crack in the roof of a house in the District to remove a branch.

A treetop fell onto power lines off Bladensburg Road in Northeast as a result of Wednesday night's storms.