Tony Blair has insisted he made the right decision by inviting US President George Bush to the UK as a huge security operation gets under way.
Numbers of British police officers during the visit have risen to 14,000
Some 14,000 police officers will be on duty throughout Mr Bush's four-day visit, at an estimated cost to the British taxpayer of £5m.
Tens of thousands of demonstrators, led by the Stop the War Coalition, are expected to take to the capital's streets in protest at the visit which begins on Tuesday.
The prime minister said Britain was right to stand shoulder to shoulder with the US, particularly after recent terror attacks in Turkey, Saudi Arabia, on the UN and Red Cross.
"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," Mr Blair told a CBI conference in Birmingham on Monday.
"Now is not the time to waver, now is the time to see it through."
The heightened state of security has seen extra police at ports and airports and checking people arriving on Eurostar trains from France.
However a female demonstrator managed to climb the front gates of Buckingham Palace on Monday to register her protest, before descending voluntarily and being detained by police.
Scotland Yard is in overall charge of security for Mr Bush's visit - which begins on Tuesday - and on Monday said it had increased from 5,000 to 14,000 the number of officers it will have on the streets during the visit.
Mr Bush will also be protected by hundreds of armed guards from the US.
They will not be granted diplomatic immunity, and will be subject to the British legal system if they shoot anybody, the Home Office has promised.
Mr Bush has shrugged off the prospect of protests, saying he supports free speech and expects the trip to be "fantastic".
Deputy Assistant Commissioner Andy Trotter said the Metropolitan Police's decision to use almost half its numbers of officers during the president's visit had been taken primarily because of security concerns.
"We are on a very high level of alert at the moment. We obviously have the visit of the president coinciding with that and we've got to make sure that London is kept safe and the visit goes well," he told BBC's World at One programme.
PRESIDENT BUSH'S ITINERARY
Tuesday 18 November - Arrives and receives private welcome at Buckingham Palace
Wednesday 19 November - Meets Michael Howard and Charles Kennedy, gives speech on transatlantic alliance and meets UK families of 11 Sept victims before attending royal banquet with Queen
Thursday 20 November - Meets British soldiers who fought in Afghanistan and Iraq, holds meetings with Tony Blair at Downing Street and hosts dinner at US Ambassador's residence
Friday 21 November - Travels to Tony Blair's Sedgefield constituency to meet members of the public before returning to Washington
Police had a meeting with the Stop the War Coalition, along with CND, the Muslim Association of Britain and Labour MP Jeremy Corbyn, on Monday afternoon for a final consultation on which roads can be included in the march.
Mr Corbyn emerged later saying the route was the one which they had originally asked for, having reassured police that it would be a peaceful protest.
The radical Islamist organisation, al-Muhajiroun, told the BBC it has advised Muslims to stay away because of a "real and serious risk" of a terrorist operation.
But Inayat Bunglawala, spokesman for the Muslim Council of Britain, accused Al Muhajiroun of "scaremongering".
"Al Muhajiroun never let up an opportunity to cause offence or incite
further hatred between people in this country," said Mr Bunglawala.
Charles Kennedy, leader of the UK's third largest party the Lib Dems, urged protesters to "use the opportunity to leave the president in no doubt as to the extent of public concern... about the way in which events tragically have unfolded".
He said he would be "expressing the anxieties" about the war felt by himself, his party and others in a meeting with the president.
He said he hoped there would not be a "draconian response" by the police to the demonstrations, as was seen during the visit to London by Chinese President Jiang Zemin three years ago.
As Mr Bush and his wife arrive in London on Tuesday, a Stop Bush rally will be held near Euston Station.
And on Wednesday, when the president is due at Buckingham Palace, there will be an "alternative state procession" including a Big Red Peace Bus.
London Mayor Ken Livingstone is holding a peace party in City Hall on Wednesday, attended by many groups opposed to the war in Iraq.
A protester climbed onto the Buckingham Palace gates
Mr Bush said in an interview with the BBC's Breakfast with Frost programme that he was unfazed by the planned protests.
"I understand you don't like war, and neither do I.
"But I would hope you understand that I have learned the lessons of 11 September 2001, and that terrorists declared war on the United States of America and war on people that love freedom."
Mr Bush's wife Laura defended his "wrenching" decision to go to war, stressing that it had been to ensure the security of the US and for the sake of world peace.
THE PRESIDENT'S ARMOURED CAR
1. The car is a special armour-plated version of the Cadillac deVille
2. The tyres are designed to function even if punctured
3. The exact dimensions and specifications of the car are kept secret
4. The driver is able to use night-vision technology
5. The underside of the car is also armour-plated
6. The car can carry six people