Metropolitan Police Assistant Commissioner Chris Allison, the national Olympic security co-ordinator, added without elaborating that any counter-terrorism action that had taken place was part of "business as usual", suggesting it had not been Olympics related.

"We recognise that we still have three days to go, three very important and very busy days, and therefore we won't be taking our eye off the ball," he told Reuters.

"The key for me is making sure that the Games pass off safely and 10.8 million people have a fantastic time. And that's what we are seeing at the moment...I've always said the focus should be on the sport and that is where it is."

Asked if the risk of a terrorist attack actually rose towards the end of the Games, he replied: "Potentially anything could happen at any time, and that's been the position we have taken throughout our planning and throughout our delivery."

Safety has been at the top of organisers' list of concerns ever since four young British Islamists killed 52 people in suicide bomb attacks in the capital the day after London was awarded the games in 2005.

Last month Jonathan Evans, the head of the MI5 domestic intelligence agency, warned that the Games presented an attractive target.

Policing inside the Olympic park itself is unobtrusive, with armed, uniformed police only visible at sensitive places such as train stations.

Outside the park policing is more apparent. The security operation, the biggest ever conducted in Britain in peacetime, is now in full swing, involving all sections of the armed forces from special services to the navy's biggest warship HMS Ocean which is docked in the River Thames.

Policing Minister Nick Herbert, who with Allison was visiting a premises in south London where police on Olympics duties received their daily briefing before deploying, said "collectively we are not going to let our guard down at any point. There is an ever present risk in this country and that is something we will have to be aware of."

"This has been a huge policing operation to ensure people are kept safe at the Olympics. But the Games are not over. And so it is important that operation is maintained and after that there will be the Paralympics as well."

One concern of the authorities has been to prevent any possibility of a "copycat" repeat of a 2010 attack that took place in Uganda that killed 79 people watching the final of the soccer World Cup, which was hosted by South Africa.

Asked if he was satisfied his police counterparts overseas were protecting "British pub" type venues overseas, he replied: "Yes we are working really closely with our colleagues. (UK) Counter terrorism command clearly has good links across the whole of Europe and the whole of the world and we are drawing on that."

Allison indicated there had been no Olympics-related terrorism-related arrests during the Games. He added: "The counter-terrorism command have been undertaking their investigations and they have where necessary been taking action against people, but that is part of business as usual."

(Reporting by William Maclean; editing by Jason Neely; For all the latest Olympic news go to here)

• A Permanent Economic Emergency