A rare, powerful 5.8-magnitude earthquake rattled the eastern third of the United States on Tuesday afternoon, damaging older buildings, shutting down much of the nation’s capital and unnerving tens of millions of people from New England to the Carolinas.

It was not a killer quake, nor even a particularly injurious one. But if it didn’t add up to a natural disaster, it was still a startling geological event, the strongest East Coast tremor in 67 years, and it effectively blew up the workday in Washington.

Any assumption that the region is seismically serene was corrected at 1:51 p.m. when a fault near the small town of Mineral, Va., suddenly ruptured. In Boston or Charleston or Detroit it might have felt like a sudden case of vertigo. Closer to the epicenter it was not so subtle. It began with a shudder, as if a helicopter were landing nearby or perhaps someone had turned on a large piece of machinery. Within a couple of seconds, it grew into a heaving, bucking, no-doubt-about-it earthquake.

It was over in less than a minute. Workers surged out of office buildings, and cellphone networks quickly clogged. The Federal Emergency Management Agency eventually sent out a statement asking the public to switch to e-mail or text messages.

Capstones, known as finials, fell from three spires on Washington National Cathedral, and cracks appeared in the flying buttresses on the older east side. “Please pray for the Cathedral as there has been some damage,” the cathedral said in its official Twitter feed.

An inspection turned up cracks “at the very, very top” of the Washington Monument, said National Park Service spokesman Bill Line. The 555-foot-tall stone obelisk will remain closed and “could be closed for an indefinite period of time.”

More than 500 people were displaced in Prince George’s County as authorities condemned and evacuated two high-rise apartment buildings.

The Old Soldiers’ Home had structural damage, as did the Ecuadoran Embassy. The White House and the Capitol were evacuated, as were countless Washington area office buildings. Georgetown University, the Smithsonian museums and D.C. federal courts closed.

On Tuesday evening, federal and local officials were still scrutinizing some public buildings and trying to decide whether and when to reopen.

The first warnings of the earthquake may have occurred at the National Zoo, where officials said some animals seemed to feel it coming before people did. The red ruffed lemurs began “alarm calling” a full 15 minutes before the quake hit, zoo spokeswoman Pamela Baker-Masson said. In the Great Ape House, Iris, an orangutan, let out a guttural holler 10 seconds before keepers felt the quake. The flamingos huddled together in the water seconds before people felt the rumbling. The rheas got excited. And the hooded mergansers — a kind of duck — dashed for the safety of the water.

For people, it was a lovely, sparkling day for an emergency evacuation. Much of the capital’s workforce had gathered on sidewalks by 2 p.m. The federal government later urged agencies to send non-emergency workers home.

The early evening commute degenerated into gridlock, with traffic lights out on some major streets. Metro slowed trains to 15 mph while inspecting for damage. Train service in the Northeast corridor ground to a halt temporarily while engineers examined the tracks.

The two Dominion nuclear plants in North Anna, Va., 10 miles from the epicenter, shut down automatically when the quake hit. They lost power from the grid and switched to four diesel generators, according to a spokesman at the Nuclear Regulatory Commission.

Bill Hall, spokesman for Dominion, said: “I’m in headquarters in Richmond, which is right next to a train track. I thought, ‘That train is going by awfully fast,’ and I looked and there was no train.”

Melvin Robinson Jr. of Fort Washington said he thought the earthquake was the latest gimmick from Hollywood. He was watching the new “Spy Kids” movie in 4-D at a theater on Route 1 in Alexandria with his children Camille, 11, and Melvin III, 10, when the room began shaking violently.

“We were watching the movie, and the chairs in the entire row began to shake left to right. We thought it was the special effects,” Robinson said. Someone then came on an intercom and told the moviegoers to evacuate.

The quake struck near the tail end of the second day of school for tens of thousands of Washington area students. A few minor injuries were reported in the District and Prince George’s. There was damage to some schools in both jurisdictions, as well as in Fairfax County, where the school year has not begun. Prince George’s officials announced late Tuesday that schools would be closed Wednesday to allow more time for inspections. Roosevelt and School Without Walls high schools in the District will be closed Wednesday because of damage.

Officials at the Columbia Heights Education Campus on 16th Street NW said students left the building in an orderly manner, mustering on the football field along with infants and toddlers who were wheeled in cribs from the school’s day-care center. But some students said the scene inside was panicky, with kids pushing on the staircases of the four-story building.

“It was a disaster,” said Diana Romero, a junior. “People were pushing and shoving, trying to get out. Some people were crying from the shock.”

Virginia officials were ramped up for a potential natural disaster, but they were thinking of Hurricane Irene, thousands of miles away. In Fairfax, the quake did not halt democracy: Board of Supervisors Chairman Sharon Bulova (D) vowed that a primary election would continue even if poll workers had to operate in the parking lots of closed buildings and use paper ballots.

In New York, Wall Street took a break as traders evacuated their buildings. The quake interrupted a prosecutor’s news conference about the dismissal of sexual assault charges against former International Monetary Fund head Dominique Strauss-Kahn.

The tremor was spawned in an unmapped fault near Mineral, about 87 miles southwest of Washington. The U.S. Geological Survey (USGS) received reports that the seismic waves were felt as far away as St. Louis, Montreal and Jacksonville, Fla.

“Biggest event in east since seismometers,” tweeted a College of William & Mary geologist, Chuck Bailey. He had been in a meeting with other geologists when the quake hit. They all looked at one another and realized immediately what was going on.

“It was sort of a once-in-a-lifetime earthquake. A once-in-a-century earthquake,” Bailey said.

Geologists — including USGS scientists who evacuated to the parking lot outside their Reston headquarters — warned that aftershocks may strike for several days. Three small aftershocks were reported by Tuesday evening.

“Aftershocks could go on for days, weeks or even months,” said Mike Blanpied, associate coordinator of the USGS’s earthquake hazards program.

He said it is unlikely that the 5.8-magnitude quake was a foreshock of a stronger tremor because it was near the limit of what has been experienced in this region of Virginia. The last Virginia quake of this intensity was in 1897, a tremor felt as far south as Georgia and as far west as Indiana.

“In terms of energy release, a fairly run-of-the-mill earthquake,” USGS Director Marcia McNutt wrote in an e-mail, “but because the rocks along the East coast do such a superior job of transmitting seismic energy without dissipating it, the tremor was widely felt. “

McNutt — who watched her rock hammer leap from a shelf in her Reston office — offered advice on what to do the next time the ground shakes: “Duck, get under something sturdy like a desk or a doorway, get away from falling glass. Make sure that you are not in the way of falling objects like pictures, bookshelves, books, anything that’s not firmly connected the wall.”

Californians scoffed at Washington jitters. Relaxing on the fence wall in front of the White House, Monda Tajbakhsh, 54, and her friend, just off the red-eye from San Francisco, were amused by the capital’s reaction to “a tiny 5-point something!” Tajbakhsh said. “I found it really amusing. Wolf Whatever-his-name-is was on the television!”

A group of Montgomery College students did not evacuate from the financial aid office, spokeswoman Beth Homan said. Financial aid is key before classes start next week, and they didn’t want to lose their places in line.

The sun hadn’t even gone down on this unusual Washington day before someone had found a commercial angle. A General Motors dealership on Route 355 in Rockville began advertising “aftershock price reductions!”

The following reporters and researchers contributed to coverage of the earthquake: Lauren Abdel-Razzaq, Joel Achenbach, Keith L. Alexander, Isaac Arnsdorf, Melissa Bell, Victoria Benning, Bonnie Benwick, Mark Berman, Laura Blumenfeld, Michelle Boorstein, Emma Brown, Jennifer Buske, Michael Alison Chandler, Christian Davenport, Marcia Davis, Mike DeBonis, Daniel de Vise, Karen DeYoung, Dan Eggen, Juliet Eilperin, Maggie Fazeli Fard, Mary Pat Flaherty, Elizabeth Flock, Amy Gardner, Caitlin Gibson, Ashley Halsey III, Fritz Hahn, Hamil Harris, Dana Hedgpeth, Rosalind S. Helderman, Steve Hendrix, Spencer Hsu, Jennifer Jenkins, Jenna Johnson, Paul Kane, John Kelly, Cecilia Kang, Anita Kumar, Fredrick Kunkle, Theola Labbé-DeBose, Michael Laris, Madonna Lebling, Carol D. Leonnig, Peter Marks, Ned Martel, Greg Miller, Carol A. Morello, Dan Morse, Steven Mufson, David Nakamura, Ed O’Keefe, Steven Overly, Michael E. Ruane, Jason Samenow, Robert Samuels, Matt Schudel, Lucy Shackelford, Ian Shapira, T. Rees Shapiro, Annys Shin, Delece Smith-Barrow, Jillian Sowah, Katherine Shaver, Jillian S. Sowah, Miranda S. Spivack, Nikita Stewart, Valerie Strauss, Lena Sun, Susan Svrluga, Patricia Sullivan, Paul Tenorio, Avis Thomas-Lester, Cheryl Thompson, Robert Thomson, Bill Turque, Theresa Vargas, John Wagner, Martin Weil, Gene Weingarten, Josh White, Ovetta Wiggins, Del Quentin Wilber, Clarence F. Williams, June Q. Wu, Victor Zapana and Matt Zapotosky.