WASHINGTON — The shutdown of the nation’s capital as well as other regions along the Northeastern corridor continued Thursday, after Mid-Atlantic utilities sent an urgent plea Wednesday to conserve energy and prevent widespread outages in the area.

In the biggest power emergency here in two decades, service disruptions affected 20 million to 30 million people in most of Pennsylvania, all of Delaware and the District of Columbia, parts of New Jersey, Maryland and Virginia. The plea was issued in light of the high electricity demand during the record cold weather that sapped the utility’s ability to supply power.

Mayor Sharon Pratt Kelly declared a state of emergency in the District of Columbia on Thursday, requiring all “nonessential businesses” to close and “nonessential personnel” to stay home from work.

Downtown Washington was almost a ghost town Thursday, with flagships operated by Woodward & Lothrop and Hecht & Co. closed. Nearby, the National Shoppes, which houses such tenants as The Limited and Victoria’s Secret and is usually hopping with customers, was virtually dead.

Montgomery Mall in Bethesda, Md., and Mazza Gallerie in the northwest corner of the district did not open. Other regional malls outside the city were faced with either closing or exacerbating the power shortage.

“It was a difficult decision,” said Rob Snow Den, general manager of Tyson Corner Center in McLean, Va., which, if closed, would have lost $1.5 million in sales. While the mall closed early on Wednesday, all five anchors opened on Thursday. About 75 percent of the mall’s specialty stores also opened.

While the Washington Board of Trade could not give estimates on the costs of the shutdowns, officials noted that its impact would be minimal because it occurred during a relatively slow season.

In downtown Philadelphia, all 13 units of Strawbridge & Clothier closed, but plan to reopen at noon Friday. John Wanamaker’s flagship in downtown Philadelphia closed at 3 p.m. Thursday.

Late Thursday, the Pepco utility issued a request that businesses and government agencies remain closed on Friday until 10 a.m. Most usually open between 8 a.m. and 9:30 a.m.