U.S. President Barack Obama was sworn in Sunday for a second term in office, as Washington, D.C., embarked on two days of inaugural solemnity, ceremony and celebration that will culminate in a huge public event on Monday.

Obama took the oath of office just before noon on Jan. 20 when each presidential term in office officially begins, in strict keeping with a date and time specified by the U.S. constitution.

The oath of office, which was carried live by the media, was administered by U.S. Chief Justice John Roberts at the White House.

Only a small group of family members were on hand for the simple ceremony, including first lady Michelle Obama and daughters Malia and Sasha. A few reporters were also on hand to witness the event.

Earlier in the day, U.S. Vice President Joe Biden took his oath of office. The event happened at 8:20 a.m. ET at Biden's official residence, the Naval Observatory in Washington and the swearing-in was presided over by U.S. Supreme Court judge Sonia Sotomayor.

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Now, the focus will shift to Monday, when the president, who is 51, will repeat the oath and give his inaugural speech in a massive event on the steps of the U.S. Capitol.

Some 800,000 people are expected to watch the ceremony that will be laden with pomp.

After the swearing-in, the president will then make the traditional journey, part of it on foot, down Pennsylvania Avenue to the White House.

Monday is also the holiday marking the birth of Martin Luther King, the revered U.S. civil rights leader who was assassinated in 1968.

On Saturday, Obama and First Lady Michelle Obama stained a bookcase as part of a national service event organized by the inaugural committee. Speaking at the elementary school where the event took place, Obama reminded the nation of the coming remembrance of King's birth and life.

"We think about not so much the inauguration, but we think about this is Dr. King's birthday we're going to be celebrating this weekend," the president said.

High hopes at start of first term

When Obama first took office as the 44th U.S. president, many Americans hoped the symbolism of the first black man in the White House was a turning point in the country's deeply troubled racial history.

'This is Dr. King's birthday we're going to be celebrating this weekend.'—U.S. President Barack Obama

Obama vowed to moderate the animus that was engulfing the country, but, four years later, the nation is only more divided. While Obama convincingly won a second term, the jubilation that surrounded him four years ago is subdued this time around.

Obama guided the country through many crushing challenges after taking office in 2009: ending the Iraq war, putting the Afghan war on a course toward U.S. withdrawal and saving the collapsing economy. He won approval for a sweeping health care overhaul.

Yet onerous problems remain and his success in resolving them will define his place in history. He faces fights with opposition Republicans over gun control, avoiding a default on the nation's debts, cutting the spiraling federal deficit and preventing Iran from building a nuclear weapons.

Americans increasingly see Obama as a strong leader, someone who stands up for his beliefs and is able to get things done, according to a survey by the Pew Research Center for the People & the Press.

The survey shows him with a 52 per cent job approval rating, among the highest rankings since early in his presidency. His personal favourability, 59 per cent, has rebounded from a low of 50 per cent in the 2012 campaign against Republican Mitt Romney.

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First lady Michelle Obama, center, and daughters Sasha, left, and Malia, right, at Saturday's Kids' Inaugural: Our Children. Our Future event in Washington. (Frank Franklin II/Associated Press)

Domestic issues, notably the economy and health care, dominated Obama's first term, but there were also critical international issues that could define his next four years.

Obama may have to decide whether to launch a strike against Iran's nuclear facilities, something he is loath to do. Washington and its allies believe Iran is trying to build nuclear weapons. Iran says its program is intended for producing electricity. 

Obama will also have to deal with the civil war in Syria, Israel-Palestinian tensions, a chill in relations with Russia and a series of maritime disputes in Asia. The administration has long talked of making a "pivot" toward Asia after the U.S. has directed much of its energy to the Middle East in the past decade.

Debt, budget debates looming

Yet, as Obama has begun setting the course for his second term, the political battles at home continue to dominate his attention. He faces tough opposition from Republicans, especially from among its tea party wing — lawmakers determined to shrink government and reduce the taxes.

Republicans are themselves divided between tea party loyalists adamantly opposed to compromises on taxes and spending and mainstream Republicans more open to negotiations.

A confrontation is brewing on the need for Congress to raise the limit on U.S. borrowing. A failure to reach an agreement could leave the government without money to pay its debts and lead to the first-ever U.S. default or a government shutdown.

Beyond the debt-ceiling debate are other big budget fights. Looming are automatic cuts to defence and domestic programs, originally scheduled for Jan. 1. Now they will be in late winter unless Congress and the president act. The U.S. budget runs dry in March, leading again to a potential shutdown unless both sides agree on new legislation.

Obama is also seeking new restrictions on guns and ammunition, a move avidly opposed by most Republicans and the National Rifle Association, a powerful lobbying group which believes they would violate constitutional protections for gun owners.

Among the second term's other top-tier issues, immigration may be the one in which Obama enjoys the most leverage. That's a dramatic change from his first term, when it was relegated to the background.

With files from CBC News