A security guard turn visitors away from the Smithsonian's National Air and Space Museum in Washington, April 7, 2015. [Photo/IC]

MUSEUMS EMPTIED

At least four Smithsonian museums were evacuated, including the Natural History Museum on the National Mall, where the power failure left hundreds of visitors milling around outside.

"It was just dark and a lot of people there and it was dark and it was awkward. Because, like you couldn't see anything, you're just standing there and the lights are off," tourist Karina Coulter told Reuters Television.

Power also went out as media tycoon Oprah Winfrey was speaking at a Postal Service ceremony marking the issuance of a stamp honoring poet Maya Angelou.

Some subway stations in the United States' second-busiest mass transit system ran on backup power. Buses were deployed to shuttle passengers from one station to another.

Power supplier Pepco Holdings Inc said its crews were repairing transmission equipment in Charles County, Maryland. The company's website showed a handful of customers without power by early evening, mostly in the District of Columbia.

The Southern Maryland Electric Cooperative (SMECO) said in a tweet that a Pepco transmission conductor broke loose at a SMECO substation, causing the outages. A spokeswoman for Pepco, Courtney Nogas, said a broken insulator had caused a fallen transmission line.

Thomas Popik, founder of watchdog group the Foundation for Resilient Societies, said the outage "is an excellent example of the grid's fragility and why physical security is so important."

He said federal regulations on the electric grid failed to address worries about physical security.

Admiral Bill Gortney, commander of North American Aerospace Defense Command and US Northern Command, said at a Pentagon news briefing he had been told all backup systems had kicked in.

But Gortney said he will be checking to see "did they perform as advertised? Do we need to strengthen in some areas?"