(CNN) -

The Tour de France is arguably the world's toughest event -- but it's just got a whole lot tougher.

One of the few sporting occasions where fans don't pay to watch its stars, one of the Tour's charms has been allowing its followers to get up close and personal. You are literally in touching distance of your heroes.

That's especially true on the arduous climbs in the Pyrenees and the Alps where hundreds of thousands of cycling fanatics line the roads.

But the Tour's three-day visit to Britain has raised a new potential obstacle for the cycling's elite riders -- the selfie.

As an estimated million people lined the narrow roads between Cambridge and London, the near 200-strong peloton were taking extra care to avoid spectators taking the social media craze just a little too far.

Official estimates put the number of fans who watched Le Grand Depart in Yorkshire over two days at 2.5 million, with Tour organizers claiming it was close to double.

But with most armed with mobile phone cameras, the dangers of individuals taking a shot of themselves but forgetting cyclists were powering past at 45kph per hour only inches away was all too apparent.

Team Sky star Geraint Thomas marveled at the fans supporting him as a home rider, but said the selfish selfie takers were "a giant pain in the arse."

American rider TJ Van Garderen went further and went on Twitter to send out a warning. "A dangerous mix of vanity and stupidity.

"Standing I the middle of the road with you back turned while 200 cyclists come at you, just to take a selfie. .think .TDF2014," he moaned.

Monday's stage saw one of Thomas' teammates, David Lopez, brush shoulders with a fan who was not paying attention, fortunately without serious injury to either party.

Despite being a work day, the crowds were again immense, particularly in the many small towns and villages on the route and in central London with massive fan parks in Green Park and Trafalgar Square.

Two years ago sports mad Britain was gripped by Olympic fever, now it was being replicated as the world's greatest cycling race sped past.

At the Olympic Park at Stratford in East London the crowd grew rapidly as many people took an extended lunch hour to reserve their own viewing point.

Once again most were carrying mobiles or tablet devices but there were no worries about selfies here as the roads around the Olympic Stadium were barriered -- with stewards taking their responsibilities seriously..

Malcolm Smither from Essex had been one of the much lauded Games Makers at the London Olympics, and was using his skills again as an official Tour Maker to usher the crowds to their vantage points and keep their spirits up.

"It's the first time I've been to the Tour de France and I'm very excited," he told CNN.

Fellow Tour Maker Julia Peters, from Portmouth, said it the experience had evoked memories of 2012. "It's very nostalgic for a lot of us and it is great to be back," she said.

Not all the fans were British, Tom Russell from Ohio was on holiday and as an avid cycling fan was determined to watch the Tour, "it is such a great atmosphere," he said.

But most at the side of the roadside had little knowledge of professional cycling.

Local resident Sheila Williams had taken her children and grandchildren to watch. "You've got to support them," she said while professing she had never heard of Britain's great cycling hero Bradley Wiggins.

So sure enough as first the breakaway duo of Frenchman Jean-Marc Bideau and Jan Barta of the Czech Republic came flying past, with thousand of cameras capturing the moment, Sheila and her family cheered them to an echo.

About a minute later the main bunch muscled past -- already closing fast on the doomed escapees.

The whole action had taken probably less than two minutes, after which the crowd slowly dispersed, with many them gathering around one of the big TV screens which had been put in place for the Olympics.

They saw Bideau and Barta caught after their heroic 150km effort, sportingly applauding at the moment of the catch.