More than 1 million people crammed onto the National Mall and along the inauguration parade route Tuesday to celebrate the swearing-in of the nation's first black president in what was one of the largest-ever gatherings in the nation's capital.
The Associated Press estimate is based on crowd photographs and comparisons with past events.
On the National Mall, the crowd stretched nearly two miles — from the Capitol to the steps of the Lincoln Memorial, where Martin Luther King Jr. delivered his "I Have a Dream" speech.
Most of the crowd was jammed into the area between the west front of the Capitol and the Washington Monument, where people stood shoulder-to-shoulder as Barack Obama was sworn in as the nation's 44th president. Some people complained they felt claustrophobic.
Farther away at the Lincoln Memorial, people surrounded the Reflecting Pool, and like many on the Mall they watched the inauguration on large TV screens.
10 deep on parade route
Meanwhile, spectators were lined 10 deep in some places along the 1.5-mile inaugural parade route, which began on Pennsylvania Avenue near the Capitol and ended in front of the White House. Most were dressed in heavy parkas and mittens. Hundreds also gathered on rooftops and balconies.
Garth Baylor, 54, a carpenter from Washington, D.C., said no inaugural celebration could compare to this one.
Crowds were so thick that medical personnel had trouble getting to people quickly around the Mall, District of Columbia fire and EMS department spokesman Alan Etter said. Still, he said everyone who has needed help eventually received treatment.
"Obviously the crush of people downtown is making it very challenging," Etter said. "We're doing the best we can."
At a section near the Washington Monument, fences and jersey barriers appeared to block people on all sides. People couldn't find a way out without climbing over the barriers.
The huge crowd on the Mall forced three Smithsonian Institution buildings to close temporarily because they couldn't hold any more people. Smithsonian spokeswoman Linda St. Thomas said the National Museum of American History, the Smithsonian Castle and the Hirshhorn Museum reached capacity Tuesday morning.
Tough to say goodbye
For many, getting out of the city proved more difficult than getting in.
At L'Enfant Plaza Metro station, a huge crowd waited to enter the station as National Guard troops stood on top of pedestals trying to direct traffic.
"There's no one here to tell us where to go," said Violet Smith, 53, who had traveled from Ghana with her daughter. "They could have done a better job because so many people came."
Three hours after Obama took the oath of office the line to reach the Capitol South Metro station stretched two blocks down the street. Instead of waiting, many people walked to different Metro stops but found long lines elsewhere.
For weeks, officials urged people to arrive early for the historic inauguration and throngs of revelers heeded that advice, arriving hours before daybreak.
At the Virginia Railway Express station in Fredericksburg, Va., chants of "Obama! Obama!" rang out when the line started moving at 5 a.m. for the first train into Washington. Some had been there since before 4 a.m.
"It's so energized, it's just unexplainable. Everyone is just so happy," Cindia Velasco of Los Angeles said.
The day's sense of goodwill became evident later in the day when law enforcement officials told NBC News that there had been no arrests connected with the inaugural event.
Record for D.C. transit
Lines of riders also formed in suburban parking lots for the Metro transit system, which added extra trains for the rush.
By early evening, Metrorail ridership had reached more than 930,000 people, setting an all-time high for the transit agency, officials said.
Two downtown rail stations were shut down for nearly an hour starting shortly before 9:30 a.m. after a woman fell on the tracks. She was hospitalized with non-life-threatening injuries. It was not clear how the woman ended up on the tracks, spokeswoman Candace Smith said.
Police had projected crowds ranging between 1 million and 2 million for the inauguration.
In 1981, President Ronald Reagan's inauguration drew about 500,000 people, and President Bill Clinton's 1993 inauguration drew about 800,000 people, according to National Park Service estimates.
Crowd counting has long been a controversial issue. The park service says Congress ordered it to stop doing crowd counts in 1997 after the agency was accused of underestimating numbers for the 1995 Million Man March.