"It's a massive event, people from all over the world coming here. ...There are going to be certain threats," said David McIntosh, a PSI bodyguard. "Any Royal Marine commando can adapt and transfer our skills that we've got from Iraq, from working round the Horn of Africa doing the anti-piracy stuff, and also from doing celebrity protection in Leicester Square."

The Thames has already been in the limelight this year — starting with a flotilla of some 1,000 boats that marked the Queen Elizabeth II's Diamond Jubilee in June. But the river has always been the city's spine, the core around which its fortunes were built. It's the reason London became a maritime city, then a world city.

The Thames connects great royal palaces like Windsor Castle and Hampton Court. It links seats of power like the Bank of England and the Houses of Parliament as it runs 215 miles (346 kilometers) east through London into the North Sea through the broad Thames Estuary.

It's the viewing platform to best see the city and its sights — the London Eye, the Cutty Sark clipper ship and Tower Bridge, which is at the moment adorned with a huge Olympic rings to honor the games that take place from July 27-Aug. 12.

In sporting terms, the river has long been a venue for trials of endurance, such as the annual race between rowing crews from Oxford and Cambridge universities and the Henley Royal Regatta that draws crews from all over the world.

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