BERLIN (AFP) - Barack Obama 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, in an address beamed live on US and German television channels and to viewers around the world.

"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," Obama 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."

"In Europe, the view that America is part of what has gone wrong in our world, rather than a force to help make it right, has become all too common," the 46-year-old first term senator said.

"In America, there are voices that deride and deny the importance of Europe's role in our security and our future. Both views miss the truth."

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.

In a speech that risked being seen as presumptuous, considering Obama will not even face US voters for another three months, he warned of a world where partnership was not a choice but the only means of survival.

"We cannot afford to be divided. No one nation, no matter how large or powerful, can defeat such challenges alone," he said.

He promised America under his watch would be serious about tackling global warning, a huge concern in Europe and a cause of rifts between the continent and the United States during the Bush administration.

But he also signalled he would demand Europe live up to its side of the bargain, asking for more help in the struggle against Al-Qaeda and the Taliban in Afghanistan.

"America cannot do this alone," Obama said.

"The Afghan people need our troops and your troops; our support and your support to defeat the Taliban and Al-Qaeda, to develop their economy, and to help them rebuild their nation.

"We have too much at stake to turn back now."

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