Britain will mount its biggest peacetime security operation for the London Olympics, with “lone wolf” attacks causing most concern but a range of other threats also under surveillance.
A security force of more than 40,000, backed by a huge intelligence operation, will guard venues, athletes and the millions of visitors expected to throng the British capital.
The halting of the Oxford and Cambridge University Boat Race by a protester swimming in the River Thames this month highlighted the difficulty of ruling out a disturbance at the Games.
Cyber-attacks, crowd trouble, riots, civil emergencies and even extreme weather are among the scenarios that British authorities are planning for, officials and analysts say.
The shadow of a terror attack has hung over the London Games since the start.
The day after London was named host city, four homegrown suicide bombers attacked three underground trains and a bus on July 7, 2005, killing 52 people.
It is also 40 years since Palestinian militants massacred Israeli athletes at the 1972 Munich Olympics, and 16 years after a bombing at the Atlanta games.
British Prime Minister David Cameron said during a visit by International Olympic Committee Chief Jacques Rogge last month that the security operation would be sensitive to the spirit of the Games.
Sebastian Coe, the former British athletics star who heads the London 2012 organising committee, added: “These are an Olympic Games — they are taking place in London, not siege-town. There is a balance to be struck.”
That may be, but the numbers are huge: 13,500 Ministry of Defence personnel, 12,000 police and more than 16,000 private security guards and unpaid volunteers will make up the Olympics security force.
The operation involves warplanes, two navy ships including a helicopter carrier stationed in the River Thames and batteries of surface-to-air missiles.
Britain has also ramped up its original security blueprint for the games, boosting the budget in December from £282 million to £553 million ($877 million, 662 million euros).
While the Games themselves start on July 27, the security operation will in fact get underway as early as next month when a special Scotland Yard unit shadows the Olympic Torch on its tour of the country.
But no amount of security can rule out attacks by “lone wolf” attackers with no attachment to wider terror organisations such as Al-Qaeda, who have slipped under the radar of the intelligence agencies.
Europe has been shocked by a series of such attacks, from the shootings by Islamist gunman Mohamed Merah in France in March to the bomb and gun attacks in Norway by rightwing extremist Anders Behring Breivik last year.
A more traditional threat from the Al-Qaeda network remains a possibility despite Osama bin Laden’s death in 2011, while Irish republican militants also present a risk.
A cyber attack by hackers or foreign states is a further threat, and one that could cause mass disruption if it targets Olympic or transport infrastructure.
Meanwhile as protest swimmer Trenton Oldfield proved at the Boat Race, demonstrations are a possibility.
In a major world city like London there are wider issues.
Riots are a possibility, with memories fresh of the mass unrest in London and several English cities in August 2011 which left five people dead.
London’s transport system also presents its own problems, not just because of its reputation for unreliability but also the fact that it has previously been the target of a terror attack.
In February thousands of London emergency personnel held a two-day exercise simulating a terror attack on the Underground during the Olympics.
Strange as it may sound in rainy Britain, authorities are also having to make contingency plans for the summer weather.