Obama made the call in his speech in front of the 226-foot high Victory Column, his first formal speech outside the United States.

"The walls between old allies on either side of the Atlantic cannot stand," Obama said, winning applause from over 100,000 crowds standing on the street the Berlin Wall once ran over.

The Illinois Senator urged Europe and the United States to work together to "defeat terror and dry up the well of extremism that supports it."

"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.

Obama noted that America and Europe should unite more to tackle international issues like terrorism, the Middle East and Iran.

"No one welcomes war. I recognize the enormous difficulties in Afghanistan, but my country and yours have a stake in seeing that NATO's first mission beyond Europe's borders is a success," he said.

"For the people of Afghanistan, and for our shared security, the work must be done. America cannot do this alone," he added.

Obama also said Iran should "abandon nuclear ambition" and called for a "world without nuclear weapons," which won widespread cheers from crowds.

Obama said he was speaking as a citizen, not as a president, but local media compared his speech to historic speeches in the same place by US Presidents John F. Kennedy and Ronald Reagan.