The Metropolitan Police Commissioner Sir Ian Blair has said the 2012 Olympic Games in London will be a "huge target" for terrorists.
Huge security plans are being drawn up for the Olympic Games
He said detailed security plans were being drawn up for the Games, including appointing 30,000 security officers.
All of the 200,000 workers involved in the Games will be vetted to see if they are a security risk or are working illegally in the UK.
An assistant commissioner will also be appointed as head of security.
An estimated 70,000 volunteers, 40,000 construction workers and 60,000 others will be employed for the Olympics.
Sir Ian said: "There can be no doubt that the 2012 games - if the current threat scenario stays the same - will be a huge target and we have to understand that and work on that basis.
"There is no question that hosting an Olympic Games in a liberal democracy rather than in China poses different issues."
He said it was the Met's responsibility to provide secure arrangements for a "safe and happy games".
He added that, despite a risk assessment being carried out for the Olympic bid, the 7 July bombings took place 24 hours after the Games were announced.
Assistant Commissioner Tarique Ghaffur is currently responsible for the security of the games but will hand over responsibility when a further assistant commissioner is appointed.
He said the Met would carry out security risk assessments every three months to ensure any potential new threats were identified.
"We will not tolerate criminal activity from those wanting to disrupt what is a major celebration for London, a showcase for London," he said.
He said issues outside of the terrorist threat, such as human trafficking, illegal construction workers and counterfeit operations, were also being considered but there was no specific intelligence about such operations at this stage.