WASHINGTON (AP) — The National Mall swelled into a vast, pulsing scene of expectation Tuesday as excited crowds clogged mass transit lines and security checkpoints to witness the swearing-in of President-elect Barack Obama.

Energized by the historic moment, hundreds 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, of Brooklyn, N.Y., 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., lines of riders had already 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 Capitol quickly filled with people, and security checkpoints were mobbed. The cold registered at about 20 degrees Fahrenheit at 9 a.m.

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.

By 9 a.m. the U.S. Park Police was directing viewers to the grounds of the Washington Monument, 14 blocks away from the Capitol where Obama would be sworn in.

At the Capitol, a plexiglass shield extended about two feet up from the balustrade around the speaker’s platform. Near the lectern, were seats reserved for Muhammad Ali, Elie Wiesel and Martin Luther King III. Other groups of seats were saved for past presidents, vice presidents and their spouses — the Clintons, the Gores, the Bushes and the Quayles. Each seat was furnished with a dark blue fleece blanket.

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.

Christian Alderson of Berryville, Va., went to Memphis, Tenn., 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.

Suburban subway riders also seemed to be in a jubilant mood, despite the early hour. In Fredericksburg, Va., 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.

D.C. 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. By 7 a.m., some 207,000 people had entered Washington’s Metro transit system, transit officials said. Huge lines formed outside subway stations; many parking lots filled up and had to be closed.

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

Alice Williams, a 51-year-old teacher of gifted children from Kansas City, Mo., had the coveted purple ticket that would placed her in front of the Capitol, but she got caught in the crowd bottleneck and instead was stuck a half mile away.

“We got blocked off; there was too much traffic and no guidance,” she said forlornly. “I’ve been walking for an hour and a half. All I want to do is see my president sworn in.”

The cold was also taking its toll.

Shelton Iddeen, 57, of Greensboro, N.C., arrived at the Mall and 4 a.m. and huddled in front of an ambulance to warm up .

“My hands feel really bad; you can’t feel your toes,” he said. “I’m more concerned about other people, the elderly and the young. I’ve seen a lot of people here really suffering.”

Others were unfazed.

Faosat Idowu, of Lagos, Nigeria, had tickets for the inauguration but couldn’t get through the crowds at five different entrances between the White House and Capitol Hill. She ended up walking in a highway tunnel that normally carries Interstate 395 under the Capitol grounds, closed for this one day to all but pedestrians. She wore a bright red scarf and hat adorned with dozens of green patches bearing Obama’s face and the words, “Africans for Obama.”

“It’s part of the excitement,” Idowu said. “I don’t mind it at all.”