"Tonight, we mark the end of one historic journey with the beginning of another." And with that, Barack Obama declared victory over Hillary Clinton in the Democratic nominee competition and turned his mind to the race ahead against John McCain, his Republican opponent, for the White House.

The triumph for the 46-year-old politician - the son of a black Kenyan father and a white Kansan mother - has made American history, representing a remarkable rise for a man who just four years ago was serving in the state senate of Illinois.

His achievement was recognised by Condoleezza Rice, the US Secretary of State and the top-ranking African American in George W Bush's administration.

"What we're seeing is an extraordinary expression of the fact that we the people' is beginning to mean all of us," she said, referring to the opening line of the US Constitution.

At the White House, Mr Bush offered his own congratulations on Mr Obama's "historic achievement".

Earlier at his victory celebration in Minnesota - the venue for the Republican convention in September - Mr Obama declared to 17,000 supporters: "Tonight, I can stand before you and say that I will be the Democratic nominee for president of the United States."

While the senator from Illinois broke through the 2118 delegate barrier to clinch the party nomination, many of the remaining undeclared 150 super-delegates are expected to come forward and back him in the next 48 hours.

As senior Democrats called for unity against Mr McCain, Hillary Clinton, the defeated challenger, is due to meet Mr Obama by the weekend to discuss the issue of the vice presidential nomination.

While her supporters insist she deserves to be the running mate, others feel she is too divisive.

However, Terry McAuliffe, her campaign manager, made clear: "What she has always said is I will do anything to help win in the fall'."

Later, it emerged that Mr Obama had appointed a three-person team, including Caroline Kennedy, daughter of the late President John F Kennedy, to lead his search for a running mate.

Following his declaration of victory, Mr Obama had a brief conversation with Mrs Clinton after which he said: "I'm very confident of how we're going to be able to bring the party together."

Last night in Baton Rouge, Louisiana, an unconventional contest took yet another new departure when Mr McCain, the 71-year-old Vietnam veteran, challenged his opponent to accept a fresh approach in the contest to become America's 44th president.

He suggested holding at least 10 face-to-face town hall meetings with Mr Obama over summer, starting in New York next week.

"I propose these town hall meetings be as free from the regimented trappings, rules and spectacle of formal debates as possible and that we pledge to the American people we will not allow the idea to die on the negotiation table as our campaigns work out the details," the Republican Senator for Arizona wrote in a letter to his Democratic opponent.

"I also suggest we fly together to the first town hall meeting as a symbolically important act embracing the politics of civility," he suggested.

Mr McCain added: "I don't think we need any big media-run production, no process question from reporters, no spin rooms. Just two Americans running for office in the greatest nation on earth, responding to the questions of the people whose trust we must earn."