Before an enormous crowd, Democratic presidential contender Barack Obama on Thursday 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 greatest danger is to allow new walls to divide us from one another," Obama said. "The walls between old allies on either side of the Atlantic cannot stand. 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 Jews cannot stand. These now are the walls we must tear down."

Obama gave his speech in front of the Tiergarten's 226-foot high Victory Column, not far from where the Berlin Wall once divided the city.

Obama's remarks inevitably invited comparison to historic speeches in the same city by Presidents John F. Kennedy and Ronald Reagan, and he borrowed rhetoric from his own appeals to campaign audiences in the likes of Berlin, N.H., when he addressed a crowd in one of the great cities of Europe.

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

Obama's speech was the centerpiece of a fast-paced tour through Europe designed to reassure skeptical voters back home about his ability to lead the country and take a frayed cross-Atlantic alliance in a new direction after eight years of the Bush administration.

Obama and German Chancellor Angela Merkel discussed the wars in Iraq and Afghanistan as well as climate and energy issues at Germany's chancellery Thursday, part of a tour aimed at lifting the first-term senator's international standing.

Obama's 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.

Aboard his chartered campaign jet, Obama told reporters, "Hopefully, it will be viewed as a substantive articulation of the relationship I would like to see between the United States and Europe. ... I am hoping to communicate across the Atlantic the value of that relationship and how we need to build on that."

Not surprisingly, Obama 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 said, "but obviously Berlin is representative of the extraordinary success of the post-World War II effort to bring the continent together and bring the West together and then later to bring the East and the West together. And so I think it is a natural place to talk about."

Obama flew to Berlin from the Middle East, where he had toured the Holy Land and met with leaders of Israel and the Palestinians. A column of black BMW and Mercedes-Benz cars ferried Obama from the Tegel Airport to the chancellery, which sits across from the city's famed glass-domed Reichstag.

Overhead, a police helicopter kept watch. Some 700 police were being deployed during Obama's visit, which lasts through Friday morning.

Obama paused inside the gates of the chancellery to wave to a group of Bavarian 11th-graders whose class happened to be ending a tour of the building.

"We were really close," said an excited Michaela Schmid. "It was super, a real highlight."

Vernon Thomas, an 18-year-old from Omaha, Neb., who waited outside to see Obama, said he was astounded by the support for the Democrat in Germany.

"There are more people to see him here in Berlin than in my hometown," said Thomas, who said he has seen Obama speak twice in Omaha. "I think he's trying to show that he is capable of handling things overseas."

The chancellery is an imposing sandstone and concrete cube. The 205,000-square-foot building faces the restored Reichstag in the heart of Berlin's new government quarter. It dwarfs the White House and has more than three times the area of the French president's Elysee Palace in Paris.

Inside, Obama and Merkel shook hands and exchanged small talk before going into her office. On Wednesday, Merkel told reporters Germany will stand by its refusal to send combat troops to southern Afghanistan.

Obama's motorcade drew cheers from knots of people along his route from the government building to the Hotel Adlon, the hotel where President Bush stayed in 2002 and, later that year, where pop singer Michael Jackson drew criticism for holding his child out a window from his room.

As Obama got out of his car, one man yelled out in English, "Yes we can!" -- the senator's campaign refrain.

The hotel was closed off for 10 minutes while police checked a suspicious package. It was found to contain only a book. It was unclear whether Obama was in the building at the time.

Obama's speech at the Victory Column has symbolic value because several U.S. presidents, including Bill Clinton, have made significant addresses in Berlin.

Former German President Richard von Weizsaecker said the Obama event could help pave the way for a new trans-Atlantic relationship.

"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 added. "The appearance and the speech of Barack Obama are evidence that this preconception is false."