WASHINGTON (AFP) - Explosive revelations about US phone and Internet surveillance programmes will challenge President Barack Obama's popularity and moral authority when he lands in Europe on Monday.

Because he was not Mr George W. Bush, who was reviled in much of Europe, and thanks to a magnetic personal story and rise to power, Mr Obama wallowed in hero worship as a candidate in Europe in 2008, and on debut presidential trips.

Candidate Obama was the prophet of hope, who told 200,000 young people in Berlin pining for a new John F. Kennedy, that Europe and America must remember their destiny and "remake the world once again". Yet, at home and abroad, in the teeth of a global financial gale, Mr Obama learned that delivering change was tougher than promising it.

While he honoured a vow to end the unpopular Iraq war, and will halt Nato combat in Afghanistan next year, Mr Obama has disappointed Europeans on issues like climate change and closing Guantanamo Bay. And now, Mr Obama has been revealed as the figurehead of a secret American intelligence war that is more sweeping than anybody knew.

Europeans are among foreigners in the crosshairs of a vast phone and Internet surveillance programme run by the shadowy US National Security Agency (NSA) exposed in newspaper leaks.

The man who once vowed to moderate unpopular ex-president Bush's "war on terror" will be the programme's chief defender when he arrives in Northern Ireland for the G-8 summit and heads onward to visit Germany.

"Certainly, this is not a great PR exercise by the Americans, and this rekindles all of these bad memories of George W. Bush's second term," said Mr Michael Geary, a fellow at the Wilson Centre. "After four years, when Obama has tried to put a softer face on American power, this is certainly not going to prove very popular for the administration in Europe and for Obama as well."

NSA programmes have stirred warnings that the privacy of Europeans is being besmirched by an American Big Brother.

Chancellor Angela Merkel, raised under the snooping ears of East German government spies, has vowed to bring up the question on the telephone and Prism Internet programmes with Mr Obama in Berlin.

The White House sought to ease German fears on Friday, saying that Mr Obama will explain that NSA programmes do not target individuals, or specific conversations, but sweep up "meta-data" in a complex equation designed to snare terrorists.

It also pointedly noted that Germany was a staging point for some of the hijackers behind the Sept 11 attacks in 2001.

G-8 leaders, who know the extent to which European security agencies are cooperating with the NSA, face their own political pressure over the scope of the US spying programme.

"Privately, the Europeans will be extremely vocal on their concerns about this," said Ms Heather Conley of the Centre for Strategic and International Studies. "They will, I think, try to send a more soothing message publicly about it. Privately, I think there will be growing concerns."

Mr Ben Rhodes, a US deputy national security adviser, told reporters that Mr Obama would stress the safeguards and checks placed on the NSA programmes and would "publicly" discuss the issue with Germans.

After the exposure of the NSA intelligence programmes, Mr Obama, facing a domestic furore, said he welcomed public debate on the issue.

But he has done little to spur that debate, and for political reasons, can hardly offer Germans more insight into the programme than he has given Americans.

Still, Mr Obama has political capital to burn in Europe. He easily outshines regional leaders mired in high unemployment and economic misery.

The Transatlantic Trends survey by the German Marshall Fund of the United States last year showed 71 per cent of people in 12 European Union countries approved of Mr Obama's handling of international policies. Seventy-one per cent also backed his efforts to fight international terrorism.

Someone who is not American, but is familiar with the President's performance in summits, also said Mr Obama enjoyed high personal authority around the G-8 table. He is always well-briefed and respected, as someone who seeks out colleagues for personal chats, and wields considerable weight as the G-8's most powerful member.