Barack Obama, summoning images from the Cold War, told a Berlin audience Thursday that America and Europe cannot let new walls divide them as they face down common threats.

The presumptive Democratic presidential nominee spoke before a crowd of 200,000 people, according to police estimates. It was the first major speech of his overseas trip, and it drew his largest audience to date.

Expectations were high, and he was already under criticism from Republicans for staging what they said amounted to a campaign rally in Germany.

The Obama campaign pitched the address as a speech on trans-Atlantic relations, and the Illinois senator hit that theme hard.

"We cannot afford to be divided," Obama said, urging Americans and Europeans to renew an alliance that has soured in recent years. "... the greatest danger of all is to allow new walls to divide us from one another. The walls between old allies on either side of the Atlantic cannot stand."

Obama called on European support to help the United States battle terrorists in Afghanistan and stop the spread of nuclear weapons. He likened the battle against extremism to their fight against communism.

"This is the moment when we must defeat terror and dry up the well of extremism that supports it," he said.

Obama pointedly acknowledged and criticized the distrust that exists between some in Europe and the United States.

"In Europe, the view that America is part of what has gone wrong in our world, rather than a force to help us make it right, has become all too common. In America, there are voices that deride and deny the importance of Europe's role in our security and our future," he said.

"Both views miss the truth -- that Europeans today are bearing new burdens and taking more responsibility in critical parts of the world; and that just as American bases built in the last century still help to defend the security of this continent, so does our country still sacrifice greatly for freedom around the globe."

He said neither the U.S. nor Europe can "turn inward" as they face common challenges.

He spoke in front of the Tiergarten's 226-foot-high Victory Column. Throngs of onlookers held Obama signs and chanted "Yes We Can!" as he casually strolled to the podium at the start of his speech.

The crowd cheered when Obama talked about bringing the Iraq war to a close. And he called on Europeans to join in the fight against global warming, AIDS and human rights abuses in Africa and Asia.

"People of Berlin, people of the world, this is our moment. This is our time," he said.

Earlier, he sought to limit comparisons to famous speeches that Presidents John F. Kennedy and Ronald Reagan made in Berlin during the Cold War.

"They were presidents, I am a citizen," he told reporters ahead of the speech, claiming the event was not a political rally.

John McCain, meanwhile, visited a German restaurant in Columbus, Ohio, where he poked fun at Obama.

"I'd love to give a speech in Germany. But I'd much prefer to do it as president of the United States rather than as a candidate for president," McCain told reporters.

Indeed, Obama's speech cannot be delivered outside the context of his candidacy, said Linda Hobgood, director of the Speech Center in the Department of Rhetoric and Communication Studies at the University of Richmond.

"To get that crowd size and those backdrops ... it does give you pause. Who is he appealing to who is not going to vote for him already? Which voters will come over to him by virtue of this moment, by virtue of this speech situation? To tell you the truth, I can't come up with too many," Hobgood said, calling it "absolutely" a campaign speech.

"Would he have been there if he wasn't a candidate ... if he had lost to Senator (Hillary) Clinton?" she asked.

McCain spokesman Tucker Bounds called the event a "premature victory lap."

"Barack Obama offered eloquent praise for this country, but the contrast is clear. John McCain has dedicated his life to serving, improving and protecting America. Barack Obama spent an afternoon talking about it," he said in a statement.

The Berlin address came toward the end of a high-profile overseas trip that has taken him to Afghanistan and Iraq, and then to Jordan, Israel and the West Bank. Along the way he met with U.S. troops and several foreign dignitaries including Israeli Prime Minister Ehud Olmert, Iraqi Prime Minister Nouri al-Maliki, Palestinian Authority President Mahmoud Abbas and on Thursday German Chancellor Angela Merkel.

Their meeting featured "very open" and wide-ranging talks, Merkel spokesman Ulrich Wilhelm said in a statement issued after the hourlong session. Obama and Merkel also stressed the "great significance of close and friendly German-American relations," he said.

Other topics included Pakistan, the Middle East peace process, the trans-Atlantic economic partnership, the global economy and "the need for cooperation on the international level and in international organizations to solve important global questions," Wilhelm said.

Click here to read the full transcript of Obama's speech.

Click here to see photos of Obama in Germany.

The Associated Press contributed to this report.