The Olympic fear factor behind securing the Games

British military personnel walk past the Olympic Stadium ahead of the 2012 London Games

At 08:30 BST on Monday, Britain's top securocrats made their way from Scotland Yard, MI5, MI6, and their respective government departments to meet at the Cabinet Office in Whitehall.

Every day over the coming weeks, this meeting will open with a review of the overnight intelligence on threats to the Olympic Games - produced by a staff working round the clock - before looking at what the day ahead could bring.

The meeting sits at the pinnacle of a vast security edifice constructed to keep the Olympics safe. Seven years in the making, involving 40,000 people and at a cost of about £1bn - but is London ready?

The shadow of terrorism has hung over the Games for a reason. The day after London won its Olympic bid on 6 July 2005, suicide bombers struck the city.

After that, everyone knew these Olympics would be the first to be held in a "high threat" environment.

The number of players involved in delivering what has been called the largest peacetime security operation in British history is certainly bewildering.

In a rare interview, Home Office security and counter-terrorism head Charles Farr tells me: "We don't have a People's Liberation Army [which protected the 2008 Beijing Olympics], so inevitably you have a number of different organisations, and we have to ensure they co-ordinate really well together."

Critics have asked whether the security is all too much, pointing for instance to the missiles on rooftops in east London. Would they really be used?

There is a secret chart locked in a drawer somewhere in Whitehall, another of those involved in security tells me when I ask that question, which plots with clinical detail how many people would die if an airliner crashed into the Olympic Stadium.

It contrasts this with the lower number of fatalities if the airliner was shot down over London by a high-velocity missile - even if it crashed into residential areas.

"Would people die on the ground? Yes," the official tells me candidly.

The chart carefully outlines all the possible types of aircraft that could be used as a weapon and the damage they could do, establishing the best option to deal with any scenario - ranging from RAF Typhoon jets buzzing an aircraft to helicopters flying alongside with written signs.

At the extreme end, snipers can take out the pilot of a light aircraft, and missiles deal with anything larger.

The military says a "higher authority" would take that final decision.

What it means is the prime minister or one of the handful of cabinet ministers to whom he has delegated authority.

The military wants to give the decision maker as much time and as many options as possible. The hope is these kind of high-end capabilities are the least likely to be used.

Missiles on Blackheath

Much of the security work goes on in secret. MI5 has been busy conducting national security checks on the half a million people seeking Olympic accreditation - weeding out "a tiny number" - and hunting down possible plotters.

"It is business as usual - but on steroids," one security official tells me.

MI5 teams at their Thames House headquarters will "triage" all the leads that come in, and decide which are credible and which need to be acted on, while MI6 has been looking to detect and disrupt threats emanating from abroad - places like Pakistan or Yemen.

The plan was for MI5 to be able to cope with double, and peaks of four times, the normal number of threat reports, prioritising each according to its gravity and urgency.

So far much of the extra capacity put in place to deal with high-end, ambitious al-Qaeda plots has not been needed and the terror-threat level is one notch lower than planned for.

While the spies have seen a rise in the amount of chatter by terrorists, they have found little credible evidence of plotting. They will be hoping they have not missed anything.

Concerns remain about "lone wolves" - but the hope is good protective security at venues will stop any getting too close and provide confidence if anyone calls in with a hoax bomb threat.

That all requires confidence in those providing security.

The bid for the Games was made on the basis the unarmed "British bobby" would be its welcoming face. The police are in overall charge of the safety and security operation.

In the past year, concerns about protesters like the Occupy movement have taken up more time.

There is a right to protest, but assistant commissioner for the Olympics, Chris Allison, says it is "conditional".

"That right does not allow you to disrupt the Games," he tells me.

"If you're intending to do something to disrupt the games then we will take action against you and we will do whatever is necessary, proportionately within the law, to make sure that you don't disrupt the Games."

The highly visible nature of the security operation is not just about deterring terrorists. It is also about reassuring other countries.

No-one is more demanding than the Americans, who have run their own operation run out of an office on Victoria Street, checking up on London's progress.

In Washington, there is a zero-risk culture when it comes to threats, and American liaison officials have scrutinised security preparations.

Retaining their confidence is crucial, one official tells me, or else we will be in "a world of pain where they want to do it all themselves".

A final unpredictable danger stems from cyberspace. Officials say there is a "pretty high chance" "hacktivists" will try to disrupt some of the websites attached to the Games.

But they are confident the internal computer systems the Games depend on will be secure.

Could anyone mess with the clock for the 100m final?

"It is certainly built in a way to make that extremely difficult," Organising Committee of the Olympic and Paralympic Games chief executive Paul Deighton told me. "We would not want them to ask them to run it again would we?"

A special Technology Operations Centre has been established at Canary Wharf to deal with any cyber-attack.

The fervent hope of the organisers is this vast security machine will remain ticking along in the background, leaving the world to watch the sport.

But if the worst does happen, then the priorities are clear, according to Mr Farr.

"We have to take account of the possibility of an attack," he tells me.

"Our instructions from government are absolutely clear. In this circumstance, of course the priority must be to save and protect life, but our priority should also be to keep the Games running. The Games go on."

So are we ready? We are about to find out.

Listen to the full report on The World Service and BBC Radio 4 or download the podcast.