Swearing to "preserve, protect and defend" the US constitution, Barack Obama Tuesday became the nation's 44th -- and first black -- president.

Obama took the oath of office from Chief Justice John Roberts, resting his left hand on Abraham Lincoln's Bible and raising his right hand to deliver the words that formally made him the successor to president George W. Bush.

Under the gaze of at least one million in Washington's National Mall and many more around the world, Obama said: "I do solemnly swear that I will execute faithfully the office of president of the United States, and will to the best of my ability, preserve, protect, and defend the constitution of the United States.

"So help me God."