"America, this is our moment. This is our time. Our time to turn the page on the policies of the past. Our time to bring new energy and new ideas to the challenges we face. Our time to offer a new direction for the country we love," Obama told a crowd of 32,000 people at a St Paul hockey arena. Obama, an Illinois senator who would be the first black US president, gained enough delegates to win the bruising battle for the Democratic nomination against New York Sen. Hillary Clinton, who sought to be the first woman US president.
"Tonight we mark the end of one historic journey with the beginning of another -- a journey that will bring a new and better day to America," said Obama, who was joined on stage by his wife Michelle, with whom he shared a celebratory fist-bump.
As five months of nominating contests concluded on Tuesday night with votes in Montana and South Dakota, Obama congratulated Clinton on her win in South Dakota and praised her for inspiring millions of voters.
"We've certainly had our differences over the last sixteen months," he said. But he described Clinton as someone who had an "unyielding desire to improve the lives of ordinary Americans."
"Our party and our country are better off because of her, and I am a better candidate for having had the honor to compete with Hillary Rodham Clinton," he said.
Addressing concerns that the race has divided some Democrats, Obama said it was time for the party to unite to defeat McCain. He turned quickly to a lengthy attack on the Republican candidate, saying he is little different from unpopular President George W Bush. He took exception in particular to McCain's promise to stick with Bush's current strategy in Iraq.