More than 1 million people crammed onto the National Mall and along the Pennsylvania Avenue inauguration parade route Tuesday to witness the swearing-in of the nation's first black president.
The Associated Press estimate is based on crowd photographs and comparisons with past events.
People stood shoulder-to-shoulder at the Washington Monument, about 14 blocks away from the Capitol. The crowd was so tightly packed that some people complained they felt claustrophobic. Further away, people surrounded the Reflecting Pool, watching the inauguration of Barack Obama on large TV screens.
"Everyone's in a good mood," said Brenda Gruss, an attorney who looked on near the Smithsonian's natural history museum.
District of Columbia fire and EMS department spokesman Alan Etter said medical personnel were having trouble getting to people quickly around the National Mall because of the throngs of people, but that everyone who has needed help has eventually received treatment.
"Obviously the crush of people downtown is making it very challenging," Etter said. "We're doing the best we can."
For weeks, officials urged people to arrive early for the historic inauguration and throngs of revelers heeded that advice, arriving hours before daybreak.
Some 510,000 people had entered Washington's Metro transit system by 11 a.m., transit officials said. Huge lines formed outside subway stations; many parking lots filled up and had to be closed.
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. Metro urged passengers to stand at least two feet away from the platform edge for their safety.
Police had projected crowds ranging between 1 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 park service estimates.
Crowd counting has long been a controversial issue. The National 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.