Sean Payton, an operations analyst from Highland Ranch, Colo., said he hoped to hear "a nice eloquent speech that makes people feel good about being an American."
Republicans lent a touch of bipartisanship to the weekend.
"We always want any president to succeed, to do well, that means America does well and Americans do well," Sen. John Barrasso of Wyoming said on CNN's "State of the Union."
Obama took the oath in the White House Blue Room where portraits of Presidents John Adams, Thomas Jefferson, James Madison and John Tyler grace the walls. He placed a hand on a Bible held by his wife. His daughters stood nearby.
The nation's political divisions seemed scarcely to intrude as Obama, a Democrat, shook hands with Roberts, a Republican appointee, in a rite that renews American democracy every four years. Unlike four years ago, when Roberts stumbled verbally, the chief justice recited the oath without error.
Before the swearing-in, the president listened from a second-row pew at the 175-year-old Metropolitan African Methodist Episcopal Church as the Rev. Jonathan V. Newman asked God's blessing for the him and his family. "But also prepare him for battle ... because sometimes enemies insist on doing it the hard way," he said.
Like Obama, Biden began his day early. He attended Catholic Mass at his official residence at the U.S. Naval Observatory a few miles from the White House. Justice Sonia Sotomayor, the first Hispanic justice and an Obama appointee, administered the oath of office.
Biden then joined Obama at the cemetery, where the two men placed a wreath at the Tomb of the Unknowns and observed a moment of silence as a bugler sounded "Taps."
Late Sunday, Biden made a surprise appearance at the Green Inaugural Ball, sponsored by the National Wildlife Federation and other environmental groups. "I don't intend to let these four years go by without getting a hell of a lot done" on the environment, he said.
Associated Press writers Darlene Superville, Josh Lederman, Matthew Daly and Stacy Anderson contributed to this report.