BERLIN, Germany (CNN) -- Standing before a massive crowd in a city that once symbolized division, Sen. Barack Obama on Thursday warned about the dangers of allowing new walls to come between the United States and its allies.
Sen. Barack Obama addresses a large crowd Thursday at the Victory Column in Berlin, Germany.
"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 at Berlin's Victory Column in the Tiergarten park.
"The fall of the Berlin Wall brought new hope. But that very closeness has given rise to new dangers -- dangers that cannot be contained within the borders of a country or by the distance of an ocean," he said.
Obama said people of all nations must stand together to face challenges of the 21st century -- from terrorism to global warming to genocide.
"We cannot afford to be divided. No one nation, no matter how large or powerful, can defeat such challenges alone. None of us can deny these threats or escape responsibility in meeting them," he said.
The greatest danger, Obama said, "is to allow new walls to divide us from one another." Read a transcript of the speech
Obama called on Europeans and Americans to join together to "defeat terror and dry up the well of extremism that supports it." Video Watch Obama call for unity »
"If we could win a battle of ideas against the communists, we can stand with the vast majority of Muslims who reject the extremism that leads to hate instead of hope," he said.
Obama started his speech by introducing himself as a "proud citizen of the United States and a fellow citizen of the world."
Police estimated that more than 200,000 people came to watch the speech, according to The Associated Press. Video Watch the crowds wait for Obama »
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Robin Oakley, CNN's European political editor, said Obama enjoyed widespread popularity in Europe.
"He is one of those politicians who reaches parts other politicians don't reach," Oakley said. "After the unpopularity of George W. Bush, the world is waiting to love America again, and many see in Obama, with his youth and his optimism, somebody who can bring that about."