As you may know, mountaineering and climbing are among the most popular Basque sports. Sadly Paris is kinda flat, so the climbers had to choose this strange artificial structure, known as the Arch of Triumph:
The climbers protested for the death in obscure circumstances (stinking to state terrorism) of ETA militant Jon Anza, whose corpse was "found" 11 months after he went missing at the morgue of a hospital of Toulouse, having been there all the time.
One of the climbers fell and broke both ankles, as well as suffering cranial concussions, however his life is out of risk. Seven people were arrested.
Update (Apr 18): the person who fell down did so because a policeman, on top of a firefighters' crane, suddenly cut the rope he was hanging from. Luckily another of the climbers managed to grab the rope, getting his hands burned in the attempt, and probably that way saved the life of his comrade (source).