WASHINGTON » President Barack Obama was officially sworn into office for a second term Sunday in a small ceremony at the White House as the nation’s capital geared up for a full inauguration today.

Chief Justice John G. Roberts Jr. administered the oath to the 44th president, surrounded by only a few family members.

Obama will participate in the traditional — and much flashier — inauguration ceremony today on the steps of the U.S. Capitol, following the lead of his predecessors whose first day in office, as prescribed by the Constitution, fell on a Sunday. Up to 800,000 are expected to attend today, though the festivities have been scaled back from four years ago when 1.8 million poured into the city to witness the swearing-in of the nation’s first black president.

"Make sure you know that what we’re celebrating is not the election or swearing-in of a president," Obama said in brief remarks late Sunday at a candlelight reception with high-dollar donors at the tony National Building Museum. The reception was one of several star-studded events at the 57th inauguration celebration that serve as a thank-you for contributors helping to pick up the multimillion-dollar tab of days of festivities.

"What we’re doing is celebrating each other and celebrating this incredible nation that we call home," Obama said.

Obama kicks off a second four-year term with ambitions to overhaul the nation’s tax code, rewrite immigration law, tighten gun regulations and combat global warming.

But he faces a fractured political climate — in part fueled by a divided Congress and nation — as he combats an array of domestic and foreign challenges and goals, from boasting a still lagging economy to winding down the war in Af­ghani­stan.

Vice President Joe Biden ticked off the president’s first-term accomplishments to applause from the reception crowd: health care reform, ending the war in Iraq, support for same-sex marriage.

"He’s just getting started," Biden said. "In the weeks and months ahead, we’re going to reduce gun violence here in America. We’re going to pass comprehensive immigration reform. And we’re going to put this nation’s economy on a sustainable path to the future."

Earlier there was little fanfare — or acknowledgement of the daunting tasks that lay ahead — at the brief White House ceremony.

Wearing a dark suit and blue tie, Obama stood next to first lady Michelle Obama and daughters Sasha, 11, and Malia, 14, just before noon in the small, oval-shaped Blue Room on the first floor which boasts views of the Washington Monument. About a dozen other relatives, including Obama’s half sisters, Maya Soe­toro-Ng and Auma Obama, and Michelle Obama’s mother, Marian Robinson, and brother, Craig Robinson, watched from across the room.

"I do solemnly swear that I will faithfully execute the office of president of the United States and will to the best of my ability, preserve, protect and defend the Constitution of the United States, so help me God," Obama recited.

As he uttered the words, Obama placed his left hand on a Bible held by his wife and used by the first lady’s grandmother LaVaughn Delores Robinson, the first African-American female manager of the bookstore at Moody Bible Institute in Chicago.

Outside, many of the streets along today’s inaugural parade were closed to traffic and clogged with tourists enjoying an unseasonably warm day as they passed empty bleachers and buildings draped in red, white and blue bunting.

Kim Hargett of Marshville, N.C., said she thought it was important to be in Washington for the weekend because last year’s contest between Obama and Republican Mitt Romney was a turning point for the country.

"People who support Obama — for the most part — are a really positive group of people who are focused on working together and honoring different viewpoints," Hargett said. "I think that’s what (Obama) is about."