Tour incidents such as these are not unknown. On Friday, for instance, Bradley Wiggins was struck by a flare fired off by over-excited spectators.
One of the most dramatic incidents occurred in 2009 when triple world champion Oscar Freire and New Zealander Julian Dean came under an airgun attack while racing through the Vosges region.
Freire was hit in the thigh and Dean in the right thumb by small- calibre bullets fired by a spectator hidden in dense woodland. Freire removed the bullets himself and handed them over to the race doctor, but the damage could have been far more serious.
Eight years earlier, an enraged spectator drove his car through the race barriers at Metz and injured four people before he was arrested, with Tour staff treating the wounded on the winner's podium. Other attacks on Grand Tours included fans throwing pepper at cyclists during a Tour de France in 1999 and a British holidaymaker hurling rocks at the peloton during a Tour of Spain a few years later.
The final stage of the same tour in 1978 had to be suspended when protestors blocked the road with logs. And the 10th stage of the 2003 Tour de France was held up by an anti-globalisation demonstration.
On other occasions, such as when a terrorist group threatened to attack the 2005 Tour of Spain with rocket-launchers (a threat they didn't carry out), security is tightened but the racing continues.
In the UK, the most extreme assault of this kind happened in May 2009 when tin tacks were strewn over the road during the Etape Caledonia in Scotland, damaging bikes and emergency vehicles.