BERLIN (AP) - Appealing for a new citizen activism in the free world, President Barack Obama renewed his call on Wednesday to reduce US and Russian nuclear stockpiles and to confront climate change, a danger he called "the global threat of our time".

In a wide-ranging speech that enumerated a litany of challenges facing the world, Mr Obama said he wanted to reignite the spirit that Berlin displayed when it fought to reunite itself during the Cold War.

"Today's threats are not as stark as they were half a century ago, but the struggle for freedom and security and human dignity, that struggle goes on," Mr Obama said at the city's historic Brandenburg Gate under a bright, hot sun. "And I come here to this city of hope because the test of our time demands the same fighting spirit that defined Berlin a half-century ago."

The president called for a one-third reduction of US and Russian nuclear stockpiles, saying it is possible to ensure American security and a strong deterrent while also limiting nuclear weapons.

Mr Obama's address comes nearly 50 years after John F. Kennedy's famous Cold War speech in this once-divided city. Shedding his jacket and at times wiping away beads of sweat, the president stood behind a bullet-proof pane and read his remarks from text before a crowd of about 6,000.

It was a stark contrast to the speech he delivered in the city in 2008, when he summoned a crowd of 200,000 to embrace his vision for American leadership.

Whereas that speech soared with his ambition, this time Mr Obama came to caution his audience not to fall into self-satisfaction.

"Complacency is not the character of great nations," Mr Obama insisted.

"Today," he said, "people often come together in places like this to remember history, not to make it. Today we face no concrete walls or barbed wire."

The speech came just one week shy of the anniversary of Kennedy's famous Cold War speech in which he denounced communism with his declaration "Ich bin ein Berliner" (I am a Berliner). Mr Obama, clearly aware that he was in Kennedy's historic shadow, asked his audience to heed the former president's message.

"If we lift our eyes as President Kennedy calls us to do, then we'll recognise that our work is not yet done," he said. "So we are not only citizens of America or Germany, we are also citizens of the world."

Mr Obama spoke repeatedly of seeking "peace with justice" around the world by confronting intolerance, poverty, Middle East conflicts and economic inequality.

But even before his speech, White House aides were drawing attention to his call for nuclear reductions, casting it as the centerpiece of his address.

"Peace with justice means pursuing the security of a world without nuclear weapons, no matter how distant that dream may be," Mr Obama said.

"We can ensure the security of America and our allies and maintain a strong and credible strategic deterrent while reducing our deployed strategic nuclear weapons by up to one-third," he said.

Signaling a new effort to pick up his delayed environmental agenda, Mr Obama also issued a call to tackle climate change, an issue he has promised to make a priority since his 2008 presidential campaign.

"Peace with justice means refusing to condemn our children to a harsher, less hospitable planet," he said.

He said the US has expanded renewable energy from clean sources and is doubling automobile fuel efficiency. But he said that without more action by all countries, the world faces what he called a grim alternative of more severe storms, famine, floods, vanishing coastlines and displaced refugees.

"This is the future we must avert," he said. "This is the global threat of our time."

Among those in the audience, Doro Zinke, president of the Berlin-Brandenburg trade union federation, said she heard nothing unexpected in Mr Obama's speech.

"I think he's really got to deliver now," she said.

But others gave him credit for just coming to Berlin, five years into his presidency.

"The most important message here was that he came to Berlin and spoke to us and the world," said Catharina Haensch, a Berliner born in the communist east of the city who now works for the Fulbright Commission. "Even If it looks like he isn't able to fulfill all of his promises, you've got to keep on hoping."

Mr Obama said he intends to seek negotiated nuclear arsenal cuts with Russia, thus steering away from any unilateral US reductions. Moreover, Mr Obama said he would work with Nato allies to seek "bold reductions" in US and Russian tactical weapons in Europe.

Mr Obama could face objections among Nato countries where many strongly oppose removing US nuclear weapons because they worry that the Russians have a far greater number of tactical nuclear weapons within range of their territory.

In Moscow, Russian foreign policy aide Yuri Ushakov said that plans for any further arms reduction would have to involve countries beyond Russia and the United States.

"The situation is now far from what it was in the '60s and '70s, when only the USA and the Soviet Union discussed arms reduction," Mr Ushakov said.

Mr Obama's calls for cooperation with Moscow come at a time of tension between the US and Russia, which are supporting opposite sides in Syria's civil war. Russia also remains wary of US missile defense plans in Europe, despite US assurances that the shield is not aimed at Moscow.

Germany's foreign minister, Guido Westerwelle, is a strong advocate of nuclear disarmament and has long called for the removal of the last US nuclear weapons from German territory, a legacy of the Cold War. The Buechel Air Base in western Germany is one of a few remaining sites in Europe where they are based.

Under an agreement drawn up when they formed a coalition government in 2009, German Chancellor Angela Merkel's conservatives and Westerwelle's Free Democratic Party agreed to press Nato and Washington for the nuclear weapons to be withdrawn, but did not set any timeframe.

Nuclear stockpile numbers are closely guarded secrets in most nations that possess them, but private nuclear policy experts say no countries other than the US and Russia are thought to have more than 300.

The Federation of American Scientists estimates that France has about 300, China about 240, Britain about 225, and Israel, India and Pakistan roughly 100 each.