Barack Hussein Obama will raise his right hand and place his left on Bibles once owned by Martin Luther King and Abraham Lincoln and swear the oath of office before mustering for four years threatened by strife at home and abroad.
The 44th US president, and the first African American to hold the office, launched his second term with a private swearing-in ceremony on Sunday, before basking in the full pomp of his office with public celebrations Monday.
Obama will set the rhetorical tone for the remainder of his presidency with an inaugural address to a crowd expected to reach half a million, will headline a parade and then waltz with the first lady at glittering inaugural balls.
Bundled-up Obama supporters trekked into town to join snaking lines for Secret Service checkpoints guarding a steel-fenced secure zone around the White House and the inaugural parade route.
Three hours before the inauguration, the crowd built between the Stars and Stripes-draped US Capitol building, where Obama will take the oath, and the marble obelisk of the Washington monument, soaring into the sky.
Armored military vehicles and parked buses blocked major roads, as part of a security vice which included air and river exclusion zones, road closures and a heavy presence of police and National Guard reserve troops.
Temperatures were forecast for a relatively comfortable upper 30 degrees Fahrenheit (3 to 4 degrees Celsius), much warmer than the bitter chill that has had crowds shivering at some previous inaugurations. Though the mood was festive, as revelers crammed into coffee shops and subway trains heading downtown, Obama's second inauguration lacks the sense of historic promise and hope that greeted his first term in 2009.
Dignitaries, including former Democratic presidents Jimmy Carter and Bill Clinton, and Secretary of State Hillary Clinton, took their seats on a platform at the West Front of the Capitol.
One Obama supporter, the Reverend Ruddie Mingo, 54, donated time and money to the president's winning campaign against Republican Mitt Romney, and said inaugural festivities were less mobbed than four years ago.
"My hope is that his next four years we can get more stuff accomplished on both sides," he said.
Obama's political brand has been damaged by an exhausting first term battling the worst economic storm in decades and brutal partisan warfare with his Republican rivals, notably over taxes and spending.
The president started his day in traditional fashion for presidents on inauguration day, worshipping at the St John's Episcopal Church opposite the White House with his family.
When he returns to politics, Obama, 51, has a legacy to defend, including a historic health care law and a retrenchment from draining wars abroad, and he is vowing to make good on the promise of a fairer economy.
He signaled late Sunday, at a reception for supporters, that he would dwell on the "common good" and the "goodness, the resilience, neighborliness, the patriotism" of Americans in his address.
"What we are celebrating is not the election or the swearing in of the president, what we are doing is celebrating each other and celebrating this incredible nation that we call home," Obama said.
"And after we celebrate, let's make sure to work as hard as we can to pass on an America that is worthy not only of our past, but also of our future. "
Obama took the oath Sunday in a private ceremony in the Blue Room of the White House.
Chief Justice John Roberts, who stumbled when swearing in Obama to open his first term in 2009, slowly read each line of the oath out loud, before the president repeated phrases first intoned by George Washington, 224 years ago.
Obama hugged his wife and children Malia, 14, and Sasha, 11, before quipping: "I did it" to his youngest daughter.
Cheeky Sasha shot back: "You didn't mess up!"
The Constitution states that US presidential terms end at noon on January 20. When that date falls on a Sunday, there is a private swearing-in ceremony before public celebrations and a second oath taking the next day.
Four years on, Obama's status as the first black president in a nation born on a racial fault line almost seems like an afterthought now -- perhaps a sign of progress.
But poignantly, Obama takes his second, second term oath of office on the federal holiday marking civil rights pioneer King's birthday.
In another historic echo, Obama will become the second president to be sworn in four times -- thanks to the Roberts stumble in 2009 and his double oath duty this year, joining Democratic icon Franklin Roosevelt.
Obama faces several boiling foreign crises likely to shape his legacy.
The US confrontation with Iran is fast-headed to a critical point with the specter of military action becoming ever more real the longer diplomacy over Tehran's nuclear program remains stuck in neutral.