Saint-Etienne finished third from bottom in Ligue 1, had already drawn the first leg of their play-off 1-1 with Ligue 2's Auxerre. Sunday's second leg at their Stade Geoffroy-Guichard also ended 1-1.
With the scores 2-2, the tie went into a penalty shoot-out which Auxerre edged 5-4 with goalkeeper Donovan Leon making a crucial save to deny Ryad Boudebou.
As soon as Birama Toure converted the final penalty to win the tie, Saint-Etienne fans invaded the pitch and threw flares towards the official stand.
The players of both teams took refuge in the changing rooms but Prime Video television footage from the tunnel showed players, stewards and staff coughing from tear gas used by the police on the pitch to disperse the invaders.
Saint-Etienne have already been sanctioned several times this season for the behaviour of its supporters.