With much of downtown fenced off, security analysts expressed confidence the unprecedented precautions would protect Biden's noon EST (1700 GMT) oath of office from a major assault, like the attack on the US Capitol two weeks ago to the day that left five people dead.
Few signs of an organized plot to disrupt the inauguration have emerged, experts say, but the threat of "lone wolf" attacks, or violence carried out by radicalized individuals, was still a concern, particularly at state capitals.
As President Donald Trump left the White House for the final time on Wednesday morning, his helicopter flew over thousands of American and state flags planted in the National Mall, standing in for the hundreds of thousands of people who gathered for past inaugurations to watch the proceedings on big-screen televisions.
Near the Supreme Court, about 20 National Guard soldiers stood watch as a handful of protesters shouted anti-gay and anti-feminist slogans through a bullhorn.
Dennis Pluchinsky, a former State Department terrorism analyst, said some right-wing protesters may rally outside Washington's fenced-off central security zone or cause violence there, in a bid to tarnish Biden's message of reuniting the nation.
But he said their disruption shouldn't mar the ceremony. "There is nothing they can do to hurt Biden. I think all they are trying is to fly their flag and poison the inauguration," said Pluchinsky, who is publishing a four-volume history of anti-US extremism.
Pentagon and FBI officials are screening participating troops to avert any insider attack.
On Tuesday, Pentagon officials said a dozen National Guard members had been removed from inauguration duty after vetting that included screening for potential ties to right-wing extremism, and troubling text messages.
FLAGS NOT PEOPLE
Officials have left little to chance this year. Bridges between Virginia and downtown Washington have been closed, as have Metro stations in the central security area, which some residents have likened to the fortress-like Green Zone of central Baghdad in Iraq.
Some intercity bus services have been suspended, as have many bike-share stations and Amtrak trains running south from Union Station, which pass through a tunnel just east of the Capitol.
With inaugural balls and other parties previously canceled due to the coronavirus pandemic, the event has taken on a somber tone in Washington.
The number of troops in Washington is three times the normal number of National Guard troops, said Major General William Walker, commander of the DC National Guard.
"We're not taking any chances," Walker told NBC. "It's just to ensure the peaceful transition of presidential power."
In addition, some 2,300 law enforcement officials from across the United States have been sworn in as special deputy US marshals, supporting the overall security operation led by the US Secret Service.
Actions by Twitter and Facebook to suspend accounts calling for violence has pushed organizing activity onto unmoderated channels such as 8kun, said Daniel Jones, president of Advance Democracy, a nonprofit group that conducts public-interest research and investigations.
"The promotion of violence on these fringe channels are explicit, and include instructions on weapon modifications and combat techniques," Jones said. "The FBI appears to be taking these threats seriously, but disciplined lone wolf actors, regardless of ideology, are extremely difficult to track."