French police in Marseille were holding 18 people in custody on Sunday after hundreds of fans stormed the club's training ground on Saturday, the local prosecutor said.
The hardcore Ultras were demonstrating against the club's leadership and a poor run of results. The supporters forced their way into the Commanderie training ground, hurling flares and firecrackers.
The club's Ligue 1 match against Rennes on Saturday was postponed as a result.
Marseille condemned a "frenzy of violence" and said damage amounting to several hundred thousand euros had been caused.
All 18 being held by police in relation to the violence were adults, prosecutor Dominique Laurent said in a statement.
Marseille are seventh in Ligue 1, 14 points adrift of leaders Lyon and failed to progress from the group stage of this season's Champions League.