One of the strongest earthquakes ever recorded on the east coast of the United States shook buildings and rattled nerves and forced the evacuations of parts of the Capitol, White House and Pentagon.
There were no reports of deaths, but fire officials in Washington said there were at least some injuries.
The US Geological Survey said the quake just before 2pm local time on Tuesday registered magnitude 5.8 and was centred about 40 miles north west of Richmond, Virginia.
Two nuclear reactors at the North Anna Power Station, in the same county as the epicentre, were automatically taken off line by safety systems, said Roger Hannah, a spokesman for the US Nuclear Regulatory Commission.
The earthquake came less than three weeks before the 10th anniversary of the September 11 attacks, and in both Washington and New York it immediately triggered fears of something more sinister than a natural disaster.
President Barack Obama, who was on holiday at Martha's Vineyard, led a conference call on the earthquake with top administration officials, including the administrator of the Federal Emergency Management Agency.
At the Pentagon in Washington, a low rumbling built until the building itself was shaking, and people ran into the corridors of the complex. The shaking continued there, to shouts of "Evacuate! Evacuate!"
The Park Service closed all monuments and memorials on the National Mall, and ceiling tiles fell at Reagan National Airport outside Washington. All flights there were put on hold.
National Park Service engineers have found a crack near the top of the Washington Monument, presumably caused by the quake. Park service spokesman Bill Line said the monument will be closed indefinitely to keep the public safe. Also in Washington, the National Cathedral said cracks had appeared in the flying buttresses around the apse at one end.
More than 12 million people live close enough to the quake's epicentre to have felt shaking, according to the Geological Survey.