President Bush boarded Air Force One (search) for his trip to the United Kingdom Tuesday, where unprecedented security was in place for the thousands of anti-war protesters that were expected.

However, a new poll showed that more Britons than not welcomed the president's visit.

Bush said he'll use the three-day visit in Britain -- America's staunchest ally in the war against Iraq -- to confront widespread doubts in Britain and across Europe about the war. There are currently 9,000 British troops in Iraq, the largest non-American force in the coalition.

London's Metropolitan Police (search-- mindful both of protests and the potential for a terrorist attack -- scheduled 14,000 officer shifts to cover the state visit that ends Friday, nearly three times as many as the force had originally planned.

Bush, whose plane took off from Andrews Air Force in Maryland shortly before 8 a.m. Tuesday, will be a guest at Buckingham Palace (search) during his stay.

The Stop the War Coalition (search), the main organizers of a series of protests, raised its estimate for Thursday's march in central London from 60,000 to 100,000.

"Opposition is just snowballing," Lindsey German, a protest coalition leader, said Tuesday.

"We fully expect that over the next three days the true view of the British people will become evident," she said.

A poll published Tuesday in the Guardian newspaper, however, suggested that more Britons welcomed the visit than the number who opposed it.

Forty-three percent of those questioned in an ICM survey said Bush should visit Britain, while 36 percent said he should not. Some 62 percent agreed that America was "generally speaking, a force for good," while 15 percent thought it was "an evil empire."

Supporters of Prime Minister Tony Blair's Labor Party (searchwere slightly more likely, 51 percent, to welcome the visit than were backers of the opposition Conservative Party, 45 percent, although Conservative policies are generally more in tune with Bush's free-market conservatism.

The poll had more good news for Blair: 47 percent of those questioned said they supported the decision to go to war in Iraq, up from 38 percent in a similar poll in September. Opposition to the war was down 12 points to 41 percent.

ICM interviewed 1,000 people between Friday and Sunday, with a margin of error of plus or minus 3 percentage points.

Protest organizers won a major concession on Monday from police, who agreed to let an anti-war march on Thursday go through Parliament Square (search), passing Blair's office on Downing (searchStreet and the Houses of Parliament.

In an early skirmish between demonstrators and police, a 61-year-old woman scaled the main gate at Buckingham Palace on Monday and stayed there for two hours before being arrested.

Lindis Percy, a veteran peace campaigner, was arrested on suspicions of causing criminal damage and breaching the peace. She was released on bail.

She later told reporters she was "incredibly surprised" that she had been able to climb the gate.

Police dismissed security concerns, saying that Percy had not entered the secure area of the palace. "The policing operation regarding the visit of President Bush does not start until today," the department added.

The Associated Press contributed to this report.