Barack Obama, the first African American ever elected to the US presidency, told tens of thousands of cheering supporters in Chicago Tuesday night 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 two wars and an economy facing recession. He thanked his rival John McCain for his graciousness in defeat and reached out to the Republican's supporters with a promise to unite the country.

'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, who has long had huge support and faced high expectations from around the world, 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.'

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 a message of change and unity.

'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.