(Reuters) - A power outage hit the White House and much of the Washington area on Tuesday, snarling trains, emptying museums and cutting electricity to government buildings and the U.S. Capitol.

The Justice Department and State Department were also affected, along with the University of Maryland. Power company Pepco Holdings Inc said the outage stemmed from a dip in voltage because of transmission line trouble.

Power was briefly knocked out to the White House, delaying the daily press briefing.

The "power outage (is) affecting many parts of the city, and it affected the White House complex. We were on a backup generator and now we are back on normal power," a White House spokesman said.

The State Department's daily briefing also was suspended after power was lost. An official at the Department of Homeland Security said in an email, "At this time, there is no indication that this outage is the result of any malicious activity."

The U.S. Capitol complex operated using a backup generator before power was restored. Power also went out as media tycoon Oprah Winfrey was speaking at a U.S. Postal Service ceremony marking the issuance of a stamp honoring poet Maya Angelou.

Some subway stations in the United States' second-busiest transit system were running on backup power, according to the Washington Metropolitan Area Transit Authority.

The Smithsonian Institution said four museums, including the National Air and Space Museum, had been evacuated.

Pepco said its crews were repairing transmission equipment in Charles County, Maryland, south of Washington. The company's website showed about 1,400 customers without power, with most clustered in the District of Columbia.

The Washington Post quoted a District of Columbia homeland security official as saying an explosion at a plant operated by the Southern Maryland Electric Cooperative had caused a power surge that cut electricity to much of the capital area.

The power company did not immediately respond to requests for comment. Its website said about 1,700 customers, most of them southeast of Washington, were without power.

Wallace Loh, president of the University of Maryland, tweeted that power had been cut to the campus and Pepco was working to restore it.

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