LONDON (AP) -- Amid royal pageantry and a smattering of anti-war protesters, President Bush opened a state visit Wednesday defending the invasion of Iraq as a necessary use of military power while likening reconstruction efforts to rebuilding a shattered Europe after two world wars.

Queen Elizabeth II and Prince Philip gave a royal salute to the American leader, greeting Bush at Buckingham Palace.

As ceremonial cannon blasts from a 41-gun salute shook the palace, Bush and his wife, Laura, moved down a receiving line with the queen and prince, greeting Prime Minister Tony Blair, Foreign Secretary Jack Straw and a phalanx of military officers in formal dress. Bush and the prince then inspected a column of Coldstream Guards, with their trademark grey coats and tall, furry black hats.

Buckingham Palace, the queen's London residence, also was a focal point for demonstrators bitterly opposed to the Iraq war. They gathered behind metal barriers Wednesday, watched by large numbers of yellow-jacketed police officers.

But though the light crowd of protesters was kept several dozen yards from the gates, their chants could be heard inside the grounds as the president greeted dignitaries.

And the palace was the setting of a major embarrassment for British security services. A journalist got hired as a royal servant despite presenting bogus credentials. The Daily Mirror newspaper said its reporter, who quit the job as a royal footman Tuesday night after Bush's arrival at the palace, had full access to the queen's residence and to the president's guest room.

Later, in a speech to academics at Whitehall Palace, Bush was seeking to puncture what he views as misconceptions on this side of the Atlantic about America's use of force. He was subtly invoking Europe's history of appeasement of dictators, and the price Europeans paid for their governments' inaction, aides said.

Bush was explicitly reminding Europeans about the critical work the Allies did to set postwar Germany on the path to democracy, a process the Bush administration and the British are trying to accelerate today in Iraq.

Expanding his argument that all free countries are at risk from terrorism and that Iraq is a central front in the battle against terrorists, Bush was offering what his senior aides called a "three-pillared" argument for war as a last resort.

Blair, meanwhile, defended Britain's close relationship with the United States and the coalition's handling of the situation in Iraq. Blair has faced strong criticism from the British public for remaining America's staunchest ally in the Iraq war.

Blair said those responsible for recent attacks in Saudi Arabia, Turkey and Iraq "aren't the British, they aren't the Americans, they are these appalling terrorists linked to some of these appalling regimes."

"It really is about time we started to realize who are allies are, who our enemies are, stick with the one and fight the other," he told the House of Commons, to loud cheers.

As many as 100,000 people were preparing to march through London to protest the Iraq war and occupation, a fresh sign of the opposition that swept through much of Europe in the run-up to invasion and has deepened for many Europeans since.

Bush's visit to Britain had newspaper headlines Wednesday morning concentrating on the unprecedented level of security surrounding the visit. The Times of London's front page leading with "President strolls into Fortress Britain."

On the first full day of a 3 1/2-day trip to England, Bush was hoping to sway people here like Nina Baker, a Scottish Green Party activist from Glasgow.

"Everything about (Bush) is just deeply depressing," she said Wednesday outside Buckingham Palace. "Bush stole the presidency, Blair lied to the people, Bush led us down the path of war. They are not listening to the public."

Bush was reaching out to a second audience as well, by granting an interview to the London-based Al-Sharq Al-Awsat newspaper, which has an Arab readership. Also Wednesday, he was meeting with relatives of Britons lost in the Sept. 11, 2001 attacks.

Thursday, Bush was to sit down with family members of British soldiers killed in Iraq.