BERLIN (REUTERS) - In the city where John F. Kennedy and Ronald Reagan gave defiant Cold War speeches, President Barack Obama will call on Wednesday for a renewed spirit of activism by the West in tackling 21st century challenges from nuclear proliferation to climate change.

Fresh from a two day summit with Group of Eight leaders in Northern Ireland, Mr Obama concludes his short European sojourn with a trip to Berlin, the German capital that he last visited as a presidential candidate in 2008.

A lot has changed since then. After more than four years in office, Mr Obama has disappointed some Europeans who saw him as a friendlier and more progressive face of America compared to his divisive predecessor, Republican George W. Bush.

But the Democratic leader remains popular in Germany, and he has forged a pragmatic - if not warm - relationship with conservative Chancellor Angela Merkel, one of his closest European allies. Mr Obama's trip just months before the coming German election gives her a boost.

The president will spend his day in meetings with Ms Merkel, German President Joachim Gauck, and Peer Steinbrueck, the Social Democrat running against her this fall. Mr Obama and Ms Merkel are scheduled to give a press conference at midday local time.

The highlight of his visit, however, will be a speech at the historic Brandenburg Gate, which once stood alongside slabs of the Berlin Wall that divided the communist East and capitalist West sections of the city.

Mr Reagan exhorted Soviet leader Mikhail Gorbachev to "tear down this wall!" during a speech on the west side of the gate in 1987, and Mr Kennedy delivered his celebrated "Ich bin ein Berliner" remarks exactly 50 years ago at Schoeneberg city hall a few kilometres to the south.

Ms Merkel forbade Mr Obama, then an Illinois senator, from speaking in front of the famous landmark in 2008. He gave an address to some 200,000 jubilant fans in the adjacent Tiergarten park instead.

This year, to a crowd of some 4,000 government officials and students, he will stand in the Pariser Platz square on the east side of the Gate and call for Germans, Europeans and Americans to use their shared history of strong alliances to tackle pressing problems of the 21st century.

"This is the place where US presidents have gone to talk about the role of the free world, essentially," Mr Obama's deputy national security adviser Ben Rhodes told reporters aboard Air Force One on the flight to Berlin on Tuesday evening.

"The overarching point that he's going to make is the exact same level of citizen and national activism that was characterized in the Kennedy speech and the Cold War needs to be applied to the challenges we face now," he said.

Those challenges included nuclear arms control, climate change, counterterrorism, and promoting democratic values beyond the Western world, Mr Rhodes said. Mr Obama is expected to unveil new US measures to flight global warming in the coming weeks.

Heather Conley of the Washington-based Center for Strategic & International Studies said the speech came at a time when the United States, which initiated a "pivot" to Asia under Mr Obama, had a less defined relationship with Europe.

"We see, 50 years later, it is a more certain Germany within Europe and globally...and in some ways it's the U.S. that's entering uncertain waters about where the future of the transatlantic relationship goes from here," she said.

In their talks, Mr Obama and Mrs Merkel are expected to focus on the wars in Syria and Afghanistan, negotiations over a new EU-US trade pact, and revelations of a US spying program dubbed Prism that has upset Germans wary of government surveillance after the trauma of the Nazi Gestapo and East German Stasi secret police.

"I expect the chancellor to raise this issue and seek answers," Ruprecht Polenz, a member of Ms Merkel's conservatives and chairman of the foreign affairs committee of the Bundestag lower house, said of the Internet monitoring programme.

The issue may not cause as much friction as some had expected. Ms Merkel defended the approach in an interview on Monday, saying Washington's cyber-snooping had helped prevent attacks on German soil.

Mr Obama, who is joined by his wife Michelle and their two daughters, landed in Berlin on Tuesday evening to a red carpet and honour guard welcome. As his motorcade swept through the city's wide streets, Germans lined up to watch and wave. One carried a huge American flag.

Not everyone was happy to see the US leader, though. Media reports said anti-Obama protests could draw up to 5,000 people on Wednesday. The Pirates party, which campaigns for Internet freedom, has called for a rally at the Victory Column, where Mr Obama spoke in 2008.