BERLIN — In a highly unusual move for an American presidential candidate, Barack Obama staged a foreign policy speech Thursday before a huge overseas audience, calling for renewed trans-Atlantic cooperation to rein in Iran, fight religious extremism and terrorism, and address global warming and poverty.

"People of Berlin, people of the world, this is our moment; this is our time," the Democratic hopeful told an enthusiastic outdoor crowd, which local authorities estimated at more than 200,000.

Turning a critical eye on the United States and implicitly criticizing President Bush, Obama said, "I know my country has not perfected itself" and "we've made our share of mistakes, and there are times when our actions around the world have not lived up to our best intentions."

The Europeans roared with approval.

Speaking before sunset at the Victory Column in Berlin's Tiergarten, to a crowd that stretched close to a mile back to the Brandenburg Gate, the 46-year-old first-term senator, who is extremely popular in Western Europe, recalled the celebration after the 1989 fall of the Berlin Wall.

He warned that today "the greatest danger of all is to allow new walls to divide us from one another."

Obama implored European leaders to send more military troops to fight al-Qaida and the Taliban in Afghanistan alongside the United States.

"America cannot do this alone. The Afghan people need our troops and your troops," he said.

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

Obama and his campaign have sought to downplay the campaign aspects of the Berlin appearance and his entire foreign trip this week.

"I speak to you not as a candidate for president, but as a citizen," he said, "a proud citizen of the United States and a fellow citizen of the world."

The line itself echoed President John F. Kennedy's famous "Ich bin ein Berliner" speech here in 1963, when JFK said that "all free men, wherever they may live, are citizens of Berlin."

Obama arrived in Germany Thursday morning to much fanfare, as local television stations aired live coverage. Fans gathered in the streets for a glimpse of the U.S. Democratic presidential candidate outside his hotel and as he arrived for a meeting with Chancellor Angela Merkel.

Josephine Wagner-Quist of Heidelberg was among them.

"We hope that everything goes better in the United States," she said. "Maybe we'll have a little peace."