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In a stirring speech in Germany on Thursday, U.S. presidential candidate Barack Obama called on the world to look to Berlin as a symbol of strength and hope that persevered against the threat of communism.

Obama was speaking to thousands of people at the start of the European leg of a tour that has already seen him visit Afghanistan, Iraq, Israel and the Palestinian territories,.

"I come to Berlin as so many of my countrymen have come before, although tonight I speak to you not as a candidate for president but as a citizen, a proud citizen of the United States and a fellow citizen of the world."

That set the stage for a speech that called for renewed trans-Atlantic ties between Europe and the U.S., and across the world.

He called on the world to remember 1948 when Western allies united to airlift supplies into western Berlin, which had been cut off by Soviet forces.

"One fall day hundreds of thousands of Berliners came here ... and heard the city's mayor implore the world not to give up on freedom. There is only one possibility, he said, for us to stand together, united, until this battle is won. The people of Berlin have spoken, we have done our duty he said, and we will keep on doing our duty.

Obama added to rousing applause: "People of the world now do your duty, people of the world, look to Berlin."

Barack Obama also summoned Europeans and Americans together to "defeat terror and dry up the well of extremism that supports it" as surely as they conquered communism a generation ago.

"The walls between old allies on either side of the Atlantic cannot stand," Obama said, speaking not far from where the Berlin Wall once divided the city.

"The walls between the countries with the most and those with the least cannot stand. The walls between races and tribes, natives and immigrants, Christian and Muslim and Jew cannot stand," he said.

More than 200,000 people attended the open-air speech.

A 'phenomenon'

Stephen Szabo, executive director of Transatlantic Academy, told Â鶹´«Ã½net Thursday that the reason for Obama's popularity in Europe is his sharp contrast with U.S. President George Bush.

"He represents a break from the policies of George Bush," he said. "He represents a new generation of Americans."

He added that Obama is a real "phenomenon" in Europe and that rival John McCain would have no hope of drawing as many people to a speech.

Szabo said that German-American diplomatic relations have improved somewhat in the last years following the divisive invasion of Iraq. But he added that the German public still remains quite hostile to the Bush administration.

"There's a real possible of an improvement after January of next year," he said.

Republicans have chafed at the attention Obama's speech has drawn, although it was McCain who publicly complained that Obama needed to visit overseas.

On Thursday, McCain's campaign stop was at a German restaurant in swing-state Ohio and he said he would like to speak in Germany, but as president, not as a candidate.

From Germany, Obama will travel to France and the U.K.

The German speech was expected to receive a warm welcome in a country where 74 per cent of the population would vote for Obama if they were allowed to cast a ballot in the upcoming election, according to a recent poll.

"That is probably because many here in Germany see him as a cross, they say, between John F. Kennedy and Martin Luther King Jr. It has been Obama-mania here for quite some time," Christel Kucharz, a producer with ABC News, told Canada AM.

Obama delivered his address at the Victory Column in the same city where Kennedy in 1963 famously said "Ich bin ein Berliner" which translates to "I am a Berliner."

Seen by many as a key moment during the Cold War, the speech showed the U.S.'s support for democratic West Germany and drew strong support for Kennedy among Germans.

Kucharz, reporting from Berlin, said many Germans -- and Europeans -- see Obama as a potential American president they could rally behind.

After arriving in the country Thursday morning, Obama was greeted by German Chancellor Angela Merkel. The two then had a private meeting during which they were expected to discuss climate change, global free trade and troop commitments to Afghanistan.

Szabo said Merkel has been a bit reluctant about Obama's visit, because it has been difficult for her to look unbiased in the U.S. presidential campaign.

Obama was also scheduled to meet with Foreign Minister Frank-Walter Steinmeier.

Former presidents Ronald Reagan and Bill Clinton have also made important speeches in Berlin. Former German President Richard von Weizsaecker said the Obama event could boost flagging U.S./Europe relations.

"Kennedy said the famous sentence, 'Ich bin ein Berliner,'" von Weizsaecker told the Bild newspaper. "Obama could send the Berlin signal: America is counting on Europe for its future."

"We have long believed that nobody in America is interested in our continent any more," von Weizsaecker said. "The appearance and the speech of Barack Obama are evidence that this preconception is false."