Barack Hussein Obama became the 44th president of the United States on Tuesday before a massive crowd reveling in a moment of historical significance, and called on Americans to confront together an economic crisis that he said was caused by “our collective failure to make hard choices.”

Mr. Obama spoke just after noon to a sea of cheering people, appearing to number well over a million, who packed the National Mall from the Capitol to the Washington Monument and beyond. Four hours later, at the end of the parade route, he left his car and strolled with his wife along Pennsylvania Avenue on the final steps of a long march to the White House, holding hands and waving to cheering crowds.

In his inaugural address, Mr. Obama acknowledged the change his presidency represented, describing himself in his inaugural address as a “man whose father less than sixty years ago might not have been served at a local restaurant.” But although the crowd and the podium around him were full of elated African Americans, Mr. Obama, the first black to become president, did not dwell on that in his speech.

He spoke for about 20 minutes, after taking the oath of office on the same Bible used by Abraham Lincoln at his first inaugural in 1861, emphasizing his determination to unite Americans in confronting both the economic challenges facing him and the continuing fight against terrorism.