As Biden and Harris receive their first presidential escort to the White House, they're surrounded by a fortified Washington, D.C.

At least 25,000 National Guard members are in the District to safeguard the inaugural ceremonies after a seige on the U.S. Capitol two weeks ago heightened security concerns.

And at least a dozen Army National Guard members were removed from the inauguration security mission this week, including two members who were sent home after vetting uncovered extremist links, a Pentagon official said Tuesday.

-ABC News' Luis Martinez

The newly-anointed first and second families are at Arlington National Cemetery for a wreath-laying ceremony at the Tomb of the Unknown Soldier.

Biden and Harris are accompanied by all living former presidents -- other than now former President Donald Trump, who landed in his Florida Resort earlier in the day without greeting Biden, and former President Jimmy Carter, who spoke with Biden and sent his well wishes but was unable to travel due to coronavirus concerns.

Former President Barack Obama and Michelle Obama, former President George W. Bush and Laura Bush, and former President Bill Clinton and former Secretary of State Hillary Clinton joined now President Biden and Vice President Kamala Harris in a historic moment at the solemn site. At the start of the ceremony, a military band played the "National Anthem" and "Taps."

When Biden's motorcade arrived, now with a "46" on the limo's license plate, smoke from the traditional 21-gun salute drifted through the air.

In another customary event, Biden -- the nation's new commander in chief -- reviewed the readiness of troops from the steps of the Capitol's East Front.

Pass in Review is a military tradition in which the incoming president reviews the readiness of military troops with every branch of the military represented in the demonstration.

At the conclusion of the inauguration ceremonies on Capitol Hill, troops in Revolutionary War-era uniforms, some with instruments -- and all in masks -- marched before the first and second families to the tune of "Yankee Doodle Dandy."

Following the pass, Biden, Harris and their families headed to Arlington National Cemetery to lay a wreath at the Tomb of the Unknown Soldier. In a show of unity, former President Barack Obama and Michelle Obama, former President George W. Bush and Laura Bush, and former President Bill Clinton and former Secretary of State Hillary Clinton joined them.

In their first promenade around Capitol Hill in their new roles, President Biden and Vice President Kamala Harris were presented with gifts from congressional leaders -- a light-hearted, customary event that featured laughter and jokes in a familiar atmosphere given both Biden's and Harris' ties to the Senate.

Joint Congressional Committee on Inaugural Ceremonies Chair Sen. Amy Klobuchar, D-Minn., presented Biden and Harris with custom-made crystal vases, which the Lenox Company has crafted for the last nine inaugurations, with Biden's vase featuring the White House and Harris' featuring the Capitol. She said they "represent the hope and the faith the American people have placed in you to move our country forward."

Outgoing Senate Majority Leader Mitch McConnell opened his remarks with a light joke, saying he "had to note" that "not only did we just swear in a son and daughter of the Senate to these high offices but both these former senators skipped the House altogether."

McConnell went on to give Harris a flag that flew over the earlier swearing-in ceremony and praise her history-making moment.

"Today this flag flew over our former colleagues' inauguration as the very first female vice president of the United States, so to our very distinguished colleague, madam vice president," he said.

House Speaker Nancy Pelosi then also presented an inauguration flag to Biden, which she stressed was a "symbol of the hope, healing and the enthusiasm that you have for our country."

House Minority Leader Kevin McCarthy, R-Calif. -- who objected to election results -- presented Harris, "a proud daughter of California," with a photo of the moment she took the oath on the Capitol steps. House Majority Leader Steny Hoyer, D-Md., presented the same to Biden, invoking their shared faith and appreciation of hymns.