President-elect Obama marked his victory with a speech to a crowd of tens of thousands of people in his hometown of Chicago.

Obama described his election as a "defining moment" in the history of the Unites States, saying that "change has come to America".

The BBC’s correspondent in Washington says that in electing Barack Obama, the American people have expressed their unhappiness with the status quo and rejected their country’s historical racial divisions.

The Australian Prime Minister summed up how many people felt when he congratulated President-elect Obama.

“Forty-five years ago Martin Luther King had a dream of an America where men and women would be judged not on the colour of their skin but on the content of their character. Today what America has done is turn that dream into a reality,” said Prime Minister Kevin Rudd.

President-elect Obama will not take office until 20 January next year. However, when he does become president, he will face many serious challenges, including two foreign wars, climate change and what he has described as "the worst financial crisis in a century".

But the mood of the country is optimistic, according to most commentators, and Obama himself appears to relish the challenge.

"The road ahead will be long. Our climb will be steep. We may not get there in one year or even in one term, but America - I have never been more hopeful than I am tonight that we will get there," said the next President of the United States.