A magnitude-5.8 earthquake has rattled the east coast of the United States.
The quake centred on the state of Virginia but was felt in Washington, where the Pentagon and US Capitol were evacuated, as well as in New York.
The National Cathedral was damaged and the Washington Monument closed. There were no immediate reports of injuries.
Federal officials said two nuclear reactors had been taken offline near the epicentre of the quake but that no damage had been reported.
A total of 12 nuclear plants declared "unusual events" after the quake.
The quake struck some 84 miles (135km) from south-west of Washington, at a depth of 3.7 miles (6km), the US Geological Survey (USGS) said.
Its epicentre was about 8km (five miles) from the town of Mineral, in the state of Virginia.
An initial measurement of 5.8 was later upgraded to 5.9, but later amended back to 5.8 by the USGS.
The quake was one of the most powerful on the east coast of the US since 1897, according to USGS records, matching the strength of a tremor in the state of New York in 1944.
Three aftershocks were recorded during Tuesday, the USGS said, the first two measuring 2.8 and 2.2. The third shock, at 20:04 local time (00:04 GMT on Wednesday), was a magnitude-4.8 tremor.
Flights held
The main earthquake shook Washington DC for about 30 seconds at 13:51 on Monday (17:51 GMT), causing office buildings to sway and houses to shake.
The streets of the city filled with evacuated workers minutes after the quake, and police moved swiftly to cordon off key government institutions, restricting access to federal buildings on Capitol Hill.
Television monitors and lights swayed for about 30 seconds as the quake rumbled away, and the mobile phone network showed intermittent service for some time afterwards.
"When it started, it felt like someone was moving furniture next door," Peter Walker told the BBC from Washington.
"Things began to shake even more and so everyone rushed out into the corridor.
"The alarm went off and the building was evacuated. After an hour we were all sent home.
"There are traffic jams all over the city. The metro is really crowded and so some people are walking or cycling home."
President Barack Obama, on vacation in Martha's Vineyard, Massachusetts, was briefed on the quake by his senior national security and emergency management staff, the White House said, but told that no emergency assistance had been requested.
Cathedral damaged
Reports said the tremor was felt as far north as Boston, in North and South Carolina in the south, and as far west as Indianapolis and Detroit.
In Charlottesville, Virginia. some 35 miles from the epicentre, resident Deb Godden said: "Our top floor apartment shook like crazy and there was a big roaring noise all around us. After that there was silence."
Flights from New York's John F Kennedy and Newark airports were delayed while authorities checked for damage from the quake, but later resumed.
Flights out of Reagan National Airport in Washington were also put on hold, but also resumed normal service shortly after the quake.
The Amtrak passenger train network slowed its trains in its busy north-eastern routes, and advised passengers to expect delays.
In Washington, the Metro public transportation system was running trains at 15mph (24km/h) while workers inspected the tracks, and likewise said customers should expect delays.
Neither system reported injuries.
Reports of minor damage began to trickle in minutes after the quake struck.
At the National Cathedral in north-west Washington, the highest building in the US capital city, three pinnacles in the central tower snapped off and a fourth was leaning. The 30-storey high central tower suffered structural damage.
"It's not massive damage but it's very serious," Sam Lloyd, dean of the National Cathedral, told Associated Press.
Also, the embassy of Ecuador was reported to have suffered major damage.
9/11 procedures
Nuclear reactors in New York, New Jersey, Pennsylvania and Maryland were undamaged, the Nuclear Regulatory Commission said.
In Charleston, West Virginia, hundreds of workers left the state Capitol building.
"The whole building shook," a spokeswoman for the state Supreme Court said. "You could feel two different shakes. Everybody just kind of came out on their own."
The AFP news agency said procedures put in place after the 9/11 attacks were activated in New York when the quake hit. Police guided people to local parks and away from tall buildings.
One witness told AFP she saw a Wall Street skyscraper "shaking like a tuning fork".
Another said the 20th floor of the court building he was on "shook like mad" and that everyone was scared.
Fatima Richardson, 28, who was sitting on the steps of a courthouse said: "You could see the building moving. I was just freaking out."
'The building's shaking'
Lower Manhattan office worker Juan Ramos told AFP he was confused after giving blood.
"I saw my cup of coffee shaking but I thought nothing of it. I had just donated blood so I thought I had not recovered my equilibrium," he said.
The quake disrupted a news conference on the Dominic Strauss-Kahn affair given by Manhattan District Attorney Cyrus Vance.
"To deprive a defendant of his liberty, guilt has to be proven beyond a reasonable doubt," Mr Vance said.
"For generations, this standard has protected... What's happening?... The whole building's shaking... Okay, okay, I've been through earthquakes in Seattle... Slowly, slowly, don't rush."
