Chicago - Barack Obama, the first African American ever elected to the US presidency, told tens of thousands of cheering supporters in Chicago Wednesday, just after midnight, that a "new dawn" of US leadership had arrived.

"If there is anyone out there who still doubts that America is a place where all things are possible, who still wonders if the dream of our founders is alive in our time, who still questions the power of our democracy - tonight is your answer," he told the throngs that gathered in Grant Park.

Obama said he recognized the "enormity" of challenges that would face him upon entering the White House, including wars in Afghanistan and Iraq and an economy on the verge of recession.

Amid huge support and high expectations from around the world, Obama offered a stark warning for US enemies and pledged to work with allies.

"A new dawn of American leadership is at hand. To those who would tear the world down, we will defeat you. To those who seek peace and security, we support you," he said.

The claps and cheers that punctuated his acceptance speech seemed more like the worship style at a typical African American church than a raucous political rally for the man who will be the nation's first black president.

Several women in the crowd waved their hands and called out "That's right" and "Thank God for that" as he spoke, but for the most part they listened in rapt attention.

Obama thanked his rival John McCain, who earlier offered a gracious concession speech of his own in Phoenix, Arizona, which recognized the historic nature of Obama's victory and promised to help him unite the country.

But Obama made little mention of the historic nature of his presidency. He reached out to all races and ethnicities, and even to McCain's own supporters.

"To those Americans whose support I have yet to earn: I may not have won your vote tonight, but I hear your voices. I need your help. I will be your president too," Obama said.

Obama reprised many of the uplifting themes that got him to the steps of the White House, telling supporters that the US electorate had warmed to his message of change, unity and a new kind of politics. He vowed to deliver on those promises as president.

"Americans have sent a message to the world that we have never been just a collection of red states and blue states. We are and always will be the United States of America," he said. (dpa)