After losing a penalty shoot-out in the play-off against Auxerre, Saint-Etienne was demoted from Ligue 1.
After the defeat, Etienne fans stormed the pitch, flinging flares at the main stand and at the players’ tunnel.
Tear gas was used to disperse supporters by police armed with shields and batons.
The defeat brings an end to the 10-time French champions’ 18-year stint in the top division, as well as Auxerre’s 10-year hiatus.
After a 1-1 draw in the first leg, Hamza Sakhi of Auxerre scored first, before Mahdi Camara of Saint-Etienne equalized.
The tie was now decided by penalties. Donovan Leon, the goalie for Auxerre, stopped a penalty kick from Ryad Boudebou, allowing Birama Toure the chance to tie the game 5-4 in the shootout and win it for the Ligue 2 side.
He made no mistakes, and supporters flocked to the pitch as soon as the ball lodged in the goal.
As police attempted to calm the situation, players rushed to the safety of the locker rooms.
They were simply the most recent nasty episodes at the Stade Geoffroy-Guichard in Saint-Etienne. Following unrest at a game against Monaco last month, the club has been sanctioned multiple times this season for crowd disorder, and Sunday’s game was played with one stand shuttered.