"What an opportunity we have to change this country," said the 47-year-old chief executive, who also issued new ethics rules for his administration and hosted a reception at the presidential mansion for 200 inauguration volunteers and guests selected by an Internet lottery.
As CBS chief White House correspondent Chip Reid reports, after dancing at inaugural balls with first lady Michelle Obama past midnight, Obama entered the Oval Office for the first time as president at 8:35 a.m. He read a note of welcome left by President Bush, then he and the first lady climbed into the new heavily-armored presidential limousine bound for the national prayer service -- a tradition for new presidents since the time of George Washington.
"Grant to Barack Obama, president of the United States, and to all in authority your grace and good will. Bless them with your heavenly gifts, give them wisdom and strength to know and to do your will," prayed the Rev. Andy Stanley, one of numerous clerics from several religions to speak.
After returning to the White House, he began breaking cleanly with his predecessor's policies.
Aides circulated a draft of an executive order that would close the detention facility at Guantanamo Bay, Cuba, within a year and halt all war crimes trials in the meantime.
Closing the site "would further the national security and foreign policy interests of the United States and the interests of justice," read the draft prepared for the new president's signature. A copy was obtained by The Associated Press.
Some of the 245 detainees currently held at Guantanamo would be released, while others would be transferred elsewhere and later put on trial under terms to be determined.
Obama's Cabinet was moving closer to completion.
At the Capitol, the Senate confirmed Hillary Rodham Clinton as secretary of state after a one-day delay forced by Republicans. The vote was 94-2, and spectators seated in the galleries erupted in applause when it was announced.
Treasury-designate Timothy Geithner emerged unscathed from his confirmation hearing, apologizing for having failed to pay $34,000 in taxes earlier in the decade.
To the evident anger of Sen. Patrick Leahy, D-Vt., chairman of the Senate Judiciary Committee, Republicans on the panel invoked longstanding rules to postpone a vote on Eric Holder's appointment as attorney general.
Six Cabinet members have been confirmed so far, as have the two top officials at the Office of Management and Budget.
Obama's schedule for the day included separate sessions on the economy and the wars in Iraq and Afghanistan.
The new president has pledged to take bold steps to revive the economy, which is struggling through the worst recession since the Great Depression. Last week, he won approval to use $350 billion in leftover financial industry bailout funds.
He presided over the White House meeting on the economy as the House Appropriations Committee moved toward approval of $358 billion in new spending, part of the economic stimulus package making its way to his desk.
The new commander in chief held his first meeting in the Situation Room, where he, Vice President Joe Biden and senior military and foreign policy officials discussed war in Iraq and Afghanistan. Obama campaigned on a pledge to withdraw U.S. combat forces from Iraq within 16 months, and to beef up the commitment in Afghanistan.
The new White House press secretary, Robert Gibbs, said Obama's phone calls to leaders in the Middle East were meant to convey his "commitment to active engagement in pursuit of Arab-Israeli peace from the beginning of his term."
Gibbs also said that in conversations with Israeli, Palestinian, Egyptian and Jordanian leaders, the president emphasized he would work to consolidate the cease-fire between Israel and Hamas in Gaza.
Obama intends to name former Senate Majority leader George Mitchell as a special envoy to the region.
Not everything was new at the White House.
In the Oval Office, Obama worked at a desk built from the timbers of a British naval vessel, the HMS Resolute, and used off-and-on by presidents since the 1870s, including Bush. It also appeared that the carpet that Bush used in his second term, a yellow sunbeam design, was still in place.
If some of the furnishings remained in place, there was no doubt that the new president meant to fulfill his campaign promise of change.
"As of today, lobbyists will be subject to stricter limits than under any ... other administration in history," Obama told reporters as he signed the new rules. The restrictions included a ban on gifts by lobbyists to anyone serving in the administration.
He also imposed a pay freeze for about 100 White House aides who earn $100,000 or more. Its implementation was unclear, since none of them was on the payroll before Tuesday's noontime inauguration.
Obama and his wife also played host and hostess for a select 200 at an open house.
"Enjoy yourself, roam around," a smiling Obama told one guest.
"Don't break anything."
At least two more executive orders are expected in coming days, according to two Obama officials.
One official said the first will require all U.S. personnel to follow the U.S. Army Field Manual while questioning detainees. The manual explicitly prohibits threats, coercion, physical abuse and waterboarding, which creates the sensation of drowning and has been termed a form of torture by critics.
The second order will set up a study of interrogation methods that could be added to the Army manual, including some that may be more aggressive than those currently permitted.
The officials spoke on condition of anonymity, saying they were not authorized to discuss the orders until they are released.