Conservative leader David Cameron said Mr Obama was the first of a new generation of world leaders.
"In electing Barack Obama, America has made history and proved to the world that it is a nation eager for change," he said.
"This has been an exciting and inspirational contest with two great candidates.
"In these difficult times people everywhere are crying out for change. Barack Obama is the first of a new generation of leaders who will deliver it - he has my whole-hearted congratulations.
"This is an important moment not just for America but for the world.
"Barack Obama's victory will give people a new opportunity to look at the United States and see her for what I believe she is - a beacon of opportunity, freedom and democracy."
In America President George Bush phoned Mr Obama to congratulate him on winning the presidency.
He told him: "What an awesome night for you, your family and your supporters."
He added: "You are about to go on one of the great journeys of life. Congratulations and go enjoy yourself."
