After a Day of Crowds and Celebration, Obama Turns to Sober List of Challenges

After the inaugural ceremonies at the Capitol, Barack and Michelle Obama walked part of the way down Pennsylvania Avenue to the White House

WASHINGTON — After an emotional day of ceremony, celebration and partygoing, President Obama on Wednesday launched into a schedule unusually charged for a new president’s first full day in office but one reflecting the daunting challenges facing the new administration.

In one of his first official acts, Mr. Obama took an initial step toward realizing a promise to close the detention center in Guantánamo Bay, Cuba, by ordering an immediate halt to all pending military war crimes trials for 120 days as he reviews the handling of terror suspects.

The president also planned to meet with top military advisers to chart a way forward in Iraq and Afghanistan, and with his economic team regarding the grave financial crisis facing the country, which Mr. Obama said Tuesday was the matter “that we’re most focused on.”

At the same time, some of Mr. Obama’s most important nominees for his cabinet — Timothy F. Geithner for Treasury secretary, Eric H. Holder Jr. as attorney general and Hillary Rodham Clinton for secretary of state faced Senate action on their nominations.

An “open house” was also planned at the White House, though it was not truly open, but for a few hundred lucky souls.

To begin the day, Mr. Obama made the traditional trip to the National Cathedral for a prayer service, which was also attended by Vice President Joseph R. Biden Jr. and an array of other dignitaries including former President Bill Clinton and Mrs. Clinton.

The new president was operating on relatively little rest. Tuesday saw him in the public eye for more than 15 hours, from a prayer service at St. John’s Church near the White House early in the morning to the inauguration and subsequent parade in blisteringly cold weather, and then his attendance at all 10 official inaugural balls. He and Mrs. Obama danced their last dance just before midnight.

But even at the balls, his mind was not entirely on celebration.

“We’re going to have a lot of work,” he said in an interview with ABC News at the Neighborhood Ball, which was designed to include Washington residents who might otherwise have been excluded from the sleek post-inaugural ball scene.

“Starting tomorrow, we’ll be making a series of announcements both on domestic and on foreign policy that I think will be critical for us to act swiftly on,” Mr. Obama said. “Were not going to be able to delay — there have been a lot of things that have been pressing.”

At Guantánamo Bay, one of the military judges, Col. Patrick Parrish of the Army, issued a written order for the requested continuance in the case of Omar Khadr, a Canadian accused of killing an American soldier with a grenade in Afghanistan in 2002. Another judge was expected to rule shortly on the order from the secretary of defense, Robert M. Gates, “by order of the president.”

Mr. Obama’s order “has the practical effect of stopping the process, probably forever," said Lt. Commander William Kuebler of the Army, Mr. Khadr’s defense lawyer.

Mr. Obama has vowed to close the detention center in Cuba, although he has noted that complications involved in the trials, custody and disposition of the approximately 245 detainees there could take considerable time.