WASHINGTON -- As dawn broke in Washington Tuesday, the morning light unveiled a staggering crowd of hundreds of thousands of people massed on the National Mall stretching from the U.S. Capitol, back toward the Washington Monument and beyond.

Around 6:45 a.m. ET, the Mall swelled to near full capacity from the Capitol west to 7th Street with people standing elbow to elbow. Police began requiring people to enter the Mall at 12th Street near the Washington Monument, a mile from the point where U.S. Chief Justice John Roberts will swear-in Barack Obama as president just before noon ET.

Resembling a rock concert, a seamless array of camera flashbulbs glittered down the Mall as people tried to snap photos of the stage in front of the Capitol.

"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."

Temperatures dipped into the mid-teens in Washington overnight. But that didn't discourage inauguration-goers as they flooded the city's Metro system when it opened at 4 a.m. Extra trains were added for the expected rush. Many suburban parking lots filled up and had to be closed.

Thousands of people poured out of the Union Station and Capitol South Metro stations just after 5 a.m, surging toward the Mall to grab the best possible location.

Around 7:15 am, the Metro Police Department unexpectedly closed Independence Ave., at 7th Street to traffic.

"This is contrary to the multi-agency plan that was agreed to," said the United States Capitol Police in an advisory sent to U.S. Representatives, senators and congressional aides. "This development severely impacts the use of essential congressional support placards."

The police urged lawmakers and staff to find other routes to the Capitol.

Connie Grant of Birmingham, Alabama, said she got up at 3:30 a.m. after coming to Washington 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 didn't matter. "I sacrificed and came here. To me, this is very historic. I just wanted to be here."

Subway riders seemed to be in a jubilant mood, despite the early hour and occasional balky trains. World history teacher Calvin Adams of Arlington, Va., 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."'

By 4:15 a.m., a line of cars extending for more than a mile formed on the far right lane Interstate 66 in Virginia. The travelers were driving eastbound and trying to land a coveted parking spot at the Vienna metro. While parking spots were still plentiful, drivers were forced to travel single-file to enter the station's parking garages.

"We're prepared; we're braced," said Steven Taubenkibel, spokesman for the District's Metro mass-transit system.

Thousands of charter buses from across the country were in the District of Columbia, packing parking lots and even streets that closed Monday night to accommodate the surge of overnight visitors and day-trippers.

City and local planners have consistently warned visitors that they could expect extensive transportation delays.

On the closing list Tuesday are all inbound bridges connecting the District of Columbia and Virginia, though authorized vehicles, pedestrians and bicyclists are allowed. A sizable chunk of downtown Washington will be shut down, and other sections will not permit parking. The two subway stations near the National Mall will be closed for much of the day.

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.

FOX News' Chad Pergram and The Associated Press contributed to this report.