A record number of 14,000 police officers will be on duty in London over the next four days during the visit of US President George W Bush.
Despite the high level of security, protestors have been permitted to march outside of parliament.
Earlier, outside of Buckingham Palace a grandmother was arrested for attempting to scale the main gates, and for protesting against the US President by placing a US flag upside down on the gate.
Despite increasing public unrest about the visit, as well as political criticism, the British Prime Minister Tony Blair has defended the visit.
In a speech in Birmingham this afternoon, Mr Blair said that this was the moment to stand firm with the United States against terrorism.
President Bush arrives in London tomorrow evening and will stay at Buckingham Palace as a guest of Queen Elizabeth.
Many US security officials have been in Britain for weeks checking locations which the President will visit.
It is thought Britain's security services have received new intelligence suggesting al-Qaeda are planning an attack on London.
The organisers of a huge protest march planned for Thursday said they believe that warnings could be just propaganda to dissuade people from turning out for the protest.
They held talks with Scotland Yard today on the route of the demonstration. The march will be allowed down Whitehall.