WASHINGTON (AP) — The streets of the U.S. capital pulsed with expectation Tuesday as crowds determined to witness the swearing-in of President-elect Barack Obama overwhelmed mass transit lines and clogged security checkpoints.

Energized by the historic moment, tens of thousands of people turned this city's orderly grid of streets into a festive party scene. Ready to endure below-freezing temperatures, they streamed up from subway stations and thronged past parked buses, emergency vehicles and street vendors, bound for Pennsylvania Avenue and the National Mall for the inauguration.

"This is the culmination of two years of work," said Obama activist Akin Salawu, 34, who helped the candidate as a community organizer and Web producer. "We got on board when Obama was the little engine who could. He's like a child you've held onto. Now he's going out into the world."

By 4 a.m.(0900GMT), lines of riders formed in suburban parking lots for the Metro transit system, which opened early and put on extra trains for the expected rush. Many parking lots filled up and had to be closed.

Streets around the domed Capitol building quickly filled with people, and security checkpoints were mobbed. The cold registered at 21 degrees Fahrenheit (-6.Celsius) at 7:45 a.m (1245GMT).

Warming tents and other facilities on the Mall were late opening because traffic and crowds delayed staffers from reaching them. Ticket holders approaching the Inaugural site on Capitol Hill awaited security sweeps in a line estimated at thousands.

Connie Gran said she got up at 3:30 a.m. after coming to Washington from Alabama with a group. Three hours later she was still on 7th street waiting for police to clear the way into the Mall.

She said the wait did not matter. "I sacrificed and came here. To me, this is very historic. I just wanted to be here."

Christian Alderson went to Memphis, Tennessee, in 1968 to support the sanitation workers strike and said he was there when Martin Luther King Jr. was assassinated.

"That day was sorrowful," Alderson, 73, said as he stood near the mall. "This is a dream come true for me."

At the opposite end of town, Georgetown University students chanted "Obama!" and "Fired up Ready to Go!" as they walked down M Street toward the Mall.

A flea-market atmosphere prevailed on downtown streets, with white tents set up to sell Obama T-shirts and mugs as well as food, bottled water, snacks, scarves and footwarmers. The scent of grilled coiled sausages and steaming Chinese food greeted those who walked toward the parade route, more than six hours before Obama would pass by.

As the first waves of people began moving through security screenings, they scrambled for prime viewing spots along Pennsylvania Avenue — sitting on the curb, staking out plots of grass, or clambering on to cold metal benches.

Suburban subway riders also seemed to be in a jubilant mood, despite the early hour. In Fredericksburg, Virginia, an hour south of Washington, chants of "Obama! Obama!" rang out at a commuter rail station when the line started moving at 5 a.m. for the first trip into Washington.

World history teacher Calvin Adams said he got up extra early so he could witness history being made first-hand and teach it to his classes.

"Eventually I'll teach American history," said Adams, 23. "I'll say, 'This is how it works because I've been there, I've seen it.'"

The joyous mood was tempered only by delays and by the dashed expectations of revelers eager to get an up-close look at history.

Police have projected inaugural crowds between 1 million and 2 million. Planners say attendance could easily top the 1.2 million people who were at Lyndon B. Johnson's 1965 inauguration, the largest crowd the National Park Service has on record.

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Associated Press Writers Gillian Gaynair, Ricardo Alonso-Zaldivar, Kevin Freking, Ed Tobias, Ben Evans, Seth Borenstein and H. Josef Hebert contributed to this report.