Fans from Paris Saint-Germain's Auteuil Kop unveiled a giant "Free Palestine" banner before the kickoff of their Champions League game against Atltico Madrid on Wednesday, eight days before France take on Israel in Paris in a Nations League game.
"War on the pitch, but peace in the world," a message underneath said.
During the match, they unrolled another message that read: "Does a child's life in Gaza mean less than another?"
PSG said they had not been aware "of any plans to display such a message."
"Paris St Germain recalls that the Parc des Princes is -- and must remain -- a place of communion around a common passion for soccer and firmly opposes any message of a political nature in its stadium," the club added in a statement.
French Interior Minister Bruno Retailleau called the unveiling of the banner "unacceptable" and when asked if he would seek sanctions against PSG, Retailleau told Sud Radio: "I am not ruling out anything. I will demand explanations from PSG."
Last year, Celtic were fined 17,500 ($18,839) because of fans waving Palestinian flags during a Champions League game.
France take on Israel at the Stade de France next Thursday, with fans allowed in the 80,000-capacity stadium.
Questions have been raised on security surrounding the event in a country that has the largest Jewish community in Europe -- and the third in the world, way behind the United States and Israel -- as well as the biggest number of Muslims in Europe.
Last month, Paris police officials said the game would "of course be open to the public."
Italy played Israel in Udine amid tight security, but Belgium played their home game in Debrecen, Hungary, after their federation said that "In Belgium, no local administration deemed it possible to organise the Red Devils' home match against Israel."
