Britain has spent millions of pounds beefing up security in preparation for the Olympics and last month Jonathan Evans, head of Britain's domestic intelligence agency MI5, warned the Games presented an attractive target.

The national threat level is assessed at "substantial" - meaning an attack is a strong possibility - but that is one notch lower than it has been for most of the years following the July 2005 suicide bomb attacks in London which killed 52 people.

Security chiefs have repeatedly said they have no intelligence that the Olympics are being targeted.

However as the Games approach, commentators have suggested heightened vigilance could lead to an increase in the number of arrests.

David Anderson, Britain's Independent Reviewer of Terrorism Legislation, has said he would be watching the police carefully to check there was no over-reaction.

"We have a lot of people in intelligence agencies manning their desks, having their leave canceled, and no doubt there will be a temptation for people to use that time as the Olympics become closer to arrest people," he said in an interview with the Muslim News newspaper. "I am watching like a hawk."

(Editing by Pravin Char)