Tight security has been put in place outside the court in Noumea as the trial is under way of more than two dozen members of New Caledonia's mainly Kanak USTKE union.
They, including their leader, Gerard Jodar, are accused of impeding air traffic during last month's clashes at Noumea's domestic airport.
The defence has been trying to have the charges thrown out over partly procedural matters, with local radio saying four suspects have now been let go.
About a hundred union supporters have been outside the court building while at the same time a demonstration by several hundred people has been held in Noumea to denounce the union for the inconvenience it has caused.
After a general strike last week, the union met mediators and announced an end to their strike.
The stoppage caused disruptions across the territory as did the clash at Magenta airport which cut air links to the outer islands.