BERLIN — Addressing a huge throng in the middle of this once-divided city, Democratic presidential candidate Barack Obama yesterday asked Americans and Europeans to renew the partnership that once defeated communism to address 21st century threats that he said put the security of all nations at risk.

Obama invoked the sweep of history over the last half of the 20th century, pointing to Berlin as a symbol of what cooperation in the transatlantic alliance can do.

"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," he said.

Declaring himself a "proud citizen of the United States and a fellow citizen of the world," Obama told the audience that the world can afford for neither America nor Europe to turn inward.

"Partnership and cooperation among nations is not a choice," he said. "It is the one way, the only way, to protect our common security and advance our common humanity."

Obama spoke near the base of Victory Column in Tiergarten Park before a crowd estimated at about 200,000 — the largest of his presidential campaign.

Obama's implicit message to the audience back home was that he is the candidate of change and better positioned than rival John McCain to help restore American prestige around the world.

McCain's campaign fired back at Obama, with an adviser declaring that the Democratic candidate had taken a "premature victory lap" in Europe.

In his speech, Obama said the United States and Europe should lead the fight to "defeat terror and dry up the well of extremism that supports it," secure the world's loose nuclear weapons, confront the dangers of climate change and win the war of ideas with Islamic extremists.

"The walls between old allies on either side of the Atlantic cannot stand," Obama said. "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. These now are the walls we must tear down."

Obama touched briefly on Iraq, the issue that has most separated Europe from the United States, saying, "This is the moment when the world should support the millions of Iraqis who seek to rebuild their lives, even as we pass responsibility to the Iraqi government and finally bring this war to a close."

He said Europe and the United States must stand together against Iran and its nuclear ambitions and pleaded for help on Afghanistan, which earlier in his foreign tour he called the central front in the war on terror.