Blair, speaking to a business and community group in Birmingham, said that protesters were free to voice their opinion but that he had no regrets about inviting Bush to Britain.
"This is the right moment for us to stand firm with the United States in defeating terrorism wherever it is and delivering us safely from what I genuinely believe to be the security threat of the 21st century," he said. "Now is not the time to waver. Now is the time to see it through."
The prime minister's need to publicly justify his invitation has underscored not only how deeply unpopular Bush is in Britain but the considerable political damage the alliance has inflicted on Blair - and the further risk he is taking by connecting himself so closely to the president.
Blair was once the most popular prime minister in British history, but his approval ratings began to drop when he supported Bush during the buildup to the war in Iraq. They are the lowest they have been since he was swept into office six years ago.
"I don't think anybody can quite figure out why Blair invited him in the first place or why he could not somehow find a convenient diplomatic way out of carrying it through," said Peter North, president of Jesus College at the University of Oxford. "I cannot see how the president's visit can do anything but harm the prime minister."
Bush's visit has dominated the media here over the past week and shifted into high drive yesterday in Britain's national newspapers, most of which are overtly political and most of which strongly opposed the war.
"Chicken George," The Daily Mirror chided Bush in a front-page headline for his plans to bypass an address to Parliament, instead choosing an invitation-only forum for his only major speech here. The newspaper included an "Idiot's Guide to Great Britain," advising Bush that the sovereign is Queen Elizabeth II and helpfully adding, "That's pronounced 'the second' and not 'eleven.'" Above an article about the unprecedented security measures in London it published the headline, "Redneck Alert."
About the only newspaper to offer Bush positive words came from The Sun, a British tabloid that published an interview with the president in yesterday's editions. The Sun is far better known for its topless "Page Three Girls" and sensational celebrity interviews than for reporting on world events.
To some degree, the distaste for the Bush visit comes from the disruption in London, where streets are clogged with traffic on the most uneventful of days. Scotland Yard announced that huge swaths of London will be off-limits to protesters and everyday traffic alike, and it said the security force has been upgraded to include 14,000 police officers, up from 5,000 announced last week.
Inconsistently enough, given the country's deference to the royal family, Britons can be disdainful of world leaders who they believe are acting imperially. Blair has been pilloried for his use of an armored limousine with the pointed accusation that he was acting like the "president" of the United Kingdom. Even President Bill Clinton, wildly popular here, was criticized for the disruption his visit caused because of security concerns.
There are true bad feelings about Bush among many Britons, though, with nearly half of them saying he is not up to the job of being president and more than 60 percent disapproving of his performance in Iraq.
The failure to discover weapons of mass destruction is a bigger issue here than in the United States, and Britons are much more skeptical than Americans about allegations that Saddam Hussein was linked to international terrorism. Blair's contention before the war that the Iraqi leader could ready chemical weapons within 45 minutes has been the subject of an independent judicial investigation here, called the Hutton Inquiry, which is expected to issue its report in January.
"About the best thing that can be said for the timing of the visit is it comes before the Hutton Inquiry report," said David Leopold, professor of politics at Worcester College. "Blair has been trying everything to move the subject away from the problems in Iraq, and this visit only serves to remind people of the role the prime minister played in what has been the most unpopular war British troops have been involved with in my memory."
A major incident in Iraq during Bush's visit could hurt Blair, Leopold said, with the prospect of television news stations showing a split screen with Blair and Bush on one side while the other shows carnage in a war that the majority of Britons oppose.
American presidents are always greeted with colorful pomp, but Bush's visit is the first official state visit by a U.S. president, coming at the invitation of the queen but at the request of the prime minister, according to Buckingham Palace, where the president will stay.
His time here will be high on the pageantry the British excel at, but few of the events will be public. That is at least in part because of concerns about a terrorist attack and because tens of thousands of protesters are expected to fill London's streets Thursday, with demonstrators flocking here from several European countries. Lesser protests are expected for the duration of the visit.
Polls show Blair's drop in popularity closely associated with the war in Iraq and the problems there, but he has been widely criticized as making the so-called U.S.-British "special relationship" a one-way affair. Bush has gotten all he asks for, the criticism goes, with Blair winning no concessions on such matters as human rights, global warming and trade tariffs.
Nine Britons are believed to be held at the U.S. Navy base at Guantanamo Bay, Cuba, as "enemy combatants," a source of anger among many here and throughout the European Union. Bush's decision not to involve the United States in the Kyoto global-warming treaty has also been criticized by many here, including Blair. And the president's decision after his election to raise taxes on steel imports has brought threats of reprisals from the World Trade Organization and from members of Blair's Labor Party.
Blair does not have to stand for re-election until 2006, but, following British custom, he is expected to call for a vote in 2005. For months, at least, his most important political asset had been the lack of any real opposition, but that may have changed this month when his rival Conservative Party dumped its leader and elected Michael Howard, a hard-nosed and quick-witted political veteran expected to give Blair more trouble than the deposed leader, Iain Duncan Smith.
"The prime minister is in no danger solely because of the Bush visit," Leopold said. "The problem with the visit for Blair is the cumulative effect it can have, this constant reminder of what people see as a huge mistake."
Protests are being organized by the Stop the War Coalition, which in February drew more than 1 million people to a protest that ended in Hyde Park. Thursday's protest moves to Trafalgar Square, where a 10-foot statue of Bush is to be toppled. Yesterday, an anti-war protester got around the tightened security and climbed the front gates of Buckingham Palace. She was talked down after two hours and arrested.
Ron Kovic, the disabled U.S. Vietnam veteran whose life story was told in the film Born on the Fourth of July, was on all of Britain's news channels yesterday and delivered a petition to Blair's office asking that Bush's invitation be withdrawn.
"We will march, protest, demonstrate," Kovic said. "We want to let the world know we're outraged by this war policy, and we're very unhappy with the visit of George Bush."
Bush arrives this evening, London time. His public schedule begins tomorrow with an official greeting by the queen, followed by a speech at Whitehall Palace and then a banquet in Buckingham Palace.