US Democratic presidential hopeful Barack Obama on Thursday challenged a new generation of Americans and Europeans to tear down walls between estranged allies, races and faiths in a soaring call for global unity at an unprecedented mass campaign rally in Berlin.

The Democratic White House candidate told tens of thousands of people near the footprint of the old Berlin Wall that humanity faced a perilous turning point, and it was time to build "a world that stands as one."

"The greatest danger of all is to allow new walls to divide us from one another," said Obama, who has scorched through US politics at lightning speed to challenge Republican John McCain for the White House in November's election.

The strikingly audacious speech, in a fevered atmosphere in Berlin's famed Tiergarten, took the White House race out of US borders in a way never seen before, and was designed to portray Obama as a leader with unique global appeal.

"The walls between old allies on either side of the Atlantic cannot stand," he said, referring to festering divisions between Europe and the United States opened up by the US-led invasion of Iraq in 2003.

"The walls between the countries with the most and those with the least cannot stand," said Obama.

"The walls between races and tribes; natives and immigrants; Christian and Muslim and Jew cannot stand. These now are the walls we must tear down," he said, drawing cheers and applause.

Obama's speech was a clear echo of former US president Ronald Reagan's call to then Soviet leader Mikhael Gorbachev in Berlin in 1987 to "tear down this wall," before the fall of Communism.

Despite its soaring cadences however, the speech was short on specifics. Obama's aides said he would not talk policy as that is the job of a president but his critics will likely slam him for empty rhetoric.

The Illinois senator rebuked both his country and Europe for blaming one another for strains in their relations, but took pains to insulate himself from critics back home who doubt his patriotism.

"I also know how much I love America. I know that for more than two centuries, we have strived, at great cost and great sacrifice, to form a more perfect union; to seek, with other nations, a more hopeful world."

Obama, who has a narrow lead in most polls of the US race, but trails McCain when voters are asked who would be the most credible commander in chief, used Berlin's triumph over division and totalitarianism as a metaphor for the world he hoped to forge.

"People of the world look at Berlin, where a wall came down, a continent came together, and history proved that there is no challenge too great for a world that stands as one," Obama said.