US President Barack Obama yesterday plunged into the task of governing a country he has vowed to change, calling his economic and military leaders to meet America's daunting challenges.

A day after being sworn in as the first black president of the US, Obama was faced with the challenge of pulling out the US economy from recession and honoring his promise of withdrawing American troops from Iraq. He has said he would send more soldiers to Afghanistan, though, where the US is fighting a second war.

Obama returned to the White House from a round of inaugural balls around 1 am, and walked into the Oval Office for the first time as president around 8:30 am yesterday, presidential spokesman Robert Gibbs said.

Apart from meeting with his advisors, Obama has welcomed public visitors to the White House as the Congress scrutinizes his economic revival plan, and takes up the nominations of Hillary Rodham Clinton as secretary of state and Timothy Geithner as treasury secretary.

A new poll underscored the anticipation that has accompanied Obama into office. The Associated Press-Knowledge Network’s survey found that three out of every four people feel more optimistic about the country's future - and 30 percent of them are Republicans.

"Tonight, we celebrate. Tomorrow, the work begins ... Together, I am confident we will write the next great chapter in America's story," Obama said on Tuesday night at the Commander in Chief Ball. It was one of 10 official inaugural celebrations that kept him and first lady Michelle Obama up into the early morning hours.

The last of the four days of inaugural celebrations - a national prayer service that is a tradition dating to the time of first president George Washington - took place at the Washington National Cathedral in the morning.

Obama and his wife were to welcome hundreds of members of the public to a White House open house, part of his pledge to make the government more accessible for the public.

A meeting with his economic team has been planned to assess his approach and devise ways to move forward.

His promise to end the war in Iraq featured prominently on Obama's first day in office, as well.

According to officials, Obama was to hold a videoconference with members of the National Security Council and the US military commanders in the two war zones later yesterday.