The three-hour black-and-white drama set against the May 1968 uprisings in France was also a competitor in Venice where it took the best director award.
Meanwhile, Anthony Cordier's Douches Froides was handed the trophy for best first feature.
That film, about a trio of teenagers facing the critical issues of being 17 - or as the tag line reads, "a dramatic comedy about happiness" - had a berth in Directors' Fortnight at Cannes earlier this year.
Both wins were announced at Paris' famed Fouquet's restaurant by Cannes festival president Gilles Jacob who also heads the Louis Delluc jury.
Generally a good harbinger of what's to come when the Cesars are handed out after the New Year, the Louis Delluc prize marks the official kick-off of awards season in France.