Scotland Yard has denied coming under pressure from the White House to close large parts of central London during US President George Bush's state visit.
The president will visit for three days
The force is planning an estimated £5m security operation involving all its armed units and up to 5,000 police officers during the three-day visit next week.
London is on a high security alert as a possible terrorist target, and police say they will be trying to balance public order problems with security issues.
But they have denied any plans to make much of central London an "exclusion zone" and say any roads closed to traffic will be quickly re-opened.
Deputy Assistant Commissioner Andy Trotter said it was not the police's job to "spare the embarrassment" of the president, should tens of thousands of protesters turn out.
They will be allowed to line the route of the presidential convoy, but will not be allowed to get close to Mr Bush.
'No exclusion zone'
The Stop the War Coalition said it may mount a legal challenge against the police decision to stop it marching down Whitehall and past Parliament on 20 November.
Scotland Yard said was normal practice when Parliament was in session.
Mr Trotter said: "There is absolutely no exclusion zone in London - there has been no pressure on us to do that and we will only close roads when absolutely necessary.
"We guarantee that lawful and peaceful demonstrators will have every opportunity to make their point, we will facilitate that and we will assist them."
While police are expecting large protests, they had no specific intelligence of a direct threat against President Bush.
Mr Trotter said police would use "whatever powers are appropriate" to make arrests, including terrorism laws, but they would be used sparingly.
Protesters may take their fight to march down Whitehall to the courts
Police will be joined by 700 US security staff, many of whom will be armed.
"The bubble" - a ring of Mr Bush's own secret service agents - will surround the president although police deny they will be granted immunity from prosecution if they open fire.
Richard Barnes, deputy chairman of the Metropolitan Police Authority (MPA) said the government should pay for the operation.
He told BBC Radio 4's Today programme: "He has been invited by the government and the Queen.
"It is the nation that should pay for it, not Londoners.
Tory candidate for London mayor Steve Norris said: "Tony Blair was right in his Guildhall speech, when he said 'this is exactly the right time for President Bush to come to London'.
"Now that he is coming on an official visit, it is right that we welcome not shun him from the city."