Mr Fergus said the Greek anarchist groups, already blamed for some bombings around Athens that caused no casualties, posed the biggest risk of attack.

He said these groups retained an insatiable appetite for international media attention to their activities.

"Small bombing attacks are more symbolic late at night. That is a real possibility to happen. That is something that I think there is a reasonable likelihood of occurring," he said.

"For the major international terrorist groups, those that are intent on mass casualty attacks, they would have to be either very brave - braver than I think they area - or very stupid to try to attempt a major attack in Athens at the time of the games."

Mr Fergus said it was unlikely any of 400 United States special forces being sent to the games as part of a NATO force would be patrolling the streets of Athens.

"The very, very strong likelihood is they will remain in situ on board a naval vessel or inside a NATO air base," he said.

"All of it acts to I suppose increase the sense of dread of putting a message out there to any group or indeed deranged individual who might have in his mind to do some harm that this is really tantamount to a ring of steel around these Olympic Games."

Mr Fergus said this level of security was not witnessed at the Games in Sydney, Salt Lake City or Atlanta.