It is the scenario almost no-one wants to contemplate. In the middle of the biggest meeting of international leaders London has ever seen, combined with one of the biggest protests the city has experienced, terrorists strike.
At the front of the minds of everyone at the Metropolitan Police Counter-Terrorism Command at Scotland Yard is the date July 7 2005 when 52 people were killed by suicide bombers in the middle of the last economic summit to be held in Britain at Gleneagles in Scotland.
The aim of the police is not just to contain protestors, they also have to make sure leaders such at Gordon Brown and Barack Obama do not become terrorist targets.
Everyone is aware that the previous threat may not be the next one and a Mumbai-style attack on hotels is the latest consideration.
To help counter the threat the SAS will be ready to react to any emerging situation alongside armed police.