U.S. Department of State Warning on Travel to France.

While some of the facts of the following annoucement are true, the overall impression is misleading. Nothing has ever happened to tourists in Paris. Besides, the police are very present and would not even let you go near where the violence may take place (at the end of demonstrations). It is also fair to say that in the last demonstrations, the violence has been minimal and consisted in clashes between pockets of radical young punks and the police. Most demonstrations have actually been very peaceful.
Beisdesn it seems to me that one would be rather stupid to join a demonstration in a foreign country anyway:

This Public Announcement is issued to alert Americans to ongoing security concerns in France. Demonstrations, sometimes involving violence, have occurred and continue to occur in Paris and other large cities in France in opposition to a new law concerning employment contracts for young workers. This Public Announcement will expire on April 30, 2006.

Recent demonstrations have occurred at times in areas frequented by tourists. Some of the demonstrations may be announced, while others may be spontaneous. Police have responded by using tear gas. U.S. Embassy personnel have been advised to avoid all demonstrations and large gatherings.

The Department of State advises U.S. citizens traveling or residing in France to avoid areas where crowds are expected to gather, to exercise caution, particularly during evening and nighttime hours, and to closely follow media reports.