Saint-Etienne have "strongly condemned" crowd trouble following the club's relegation from Ligue 1 on Sunday.
Auxerre secured promotion to the top flight with a 5-4 penalty shoot-out victory after the two sides were level at 2-2 on aggregate.
There were ugly scenes after Saint-Etienne's 18-year stay in Ligue 1 was brought to an end, with fans storming onto the pitch.
Flares were thrown at the main stand, while players appeared to be targeted by supporters, who also clashed with police and security staff on the field.
Saint-Etienne vowed to take action following a dark day for the 10-time champions of France.
A club statement said: "Despite an exceptional and reinforced deployment of nearly 500 agents, many supporters invaded the pitch at the final whistle of the match against Auxerre.
"Some were guilty of several degradations and acts of violence towards players, the security agents, police and the fans in the Pierre-Faurand stand
"ASSE strongly condemns these acts, gives its full support to those affected and will initiate the necessary legal proceedings."